DEV: Chat service object initial implementation (#19814)

This is a combined work of Martin Brennan, Loïc Guitaut, and Joffrey Jaffeux.

---

This commit implements a base service object when working in chat. The documentation is available at https://discourse.github.io/discourse/chat/backend/Chat/Service.html

Generating documentation has been made as part of this commit with a bigger goal in mind of generally making it easier to dive into the chat project.

Working with services generally involves 3 parts:

- The service object itself, which is a series of steps where few of them are specialized (model, transaction, policy)

```ruby
class UpdateAge
  include Chat::Service::Base

  model :user, :fetch_user
  policy :can_see_user
  contract
  step :update_age

  class Contract
    attribute :age, :integer
  end

  def fetch_user(user_id:, **)
    User.find_by(id: user_id)
  end

  def can_see_user(guardian:, **)
    guardian.can_see_user(user)
  end

  def update_age(age:, **)
    user.update!(age: age)
  end
end
```

- The `with_service` controller helper, handling success and failure of the service within a service and making easy to return proper response to it from the controller

```ruby
def update
  with_service(UpdateAge) do
    on_success { render_serialized(result.user, BasicUserSerializer, root: "user") }
  end
end
```

- Rspec matchers and steps inspector, improving the dev experience while creating specs for a service

```ruby
RSpec.describe(UpdateAge) do
  subject(:result) do
    described_class.call(guardian: guardian, user_id: user.id, age: age)
  end

  fab!(:user) { Fabricate(:user) }
  fab!(:current_user) { Fabricate(:admin) }

  let(:guardian) { Guardian.new(current_user) }
  let(:age) { 1 }

   it { expect(user.reload.age).to eq(age) }
end
```

Note in case of unexpected failure in your spec, the output will give all the relevant information:

```
  1) UpdateAge when no channel_id is given is expected to fail to find a model named 'user'
     Failure/Error: it { is_expected.to fail_to_find_a_model(:user) }

       Expected model 'foo' (key: 'result.model.user') was not found in the result object.

       [1/4] [model] 'user' ❌
       [2/4] [policy] 'can_see_user'
       [3/4] [contract] 'default'
       [4/4] [step] 'update_age'

       /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/update_age.rb:32:in `fetch_user': missing keyword: :user_id (ArgumentError)
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:202:in `instance_exec'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:202:in `call'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:219:in `call'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:417:in `block in run!'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:417:in `each'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:417:in `run!'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:411:in `run'
       	from <internal:kernel>:90:in `tap'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:302:in `call'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/spec/services/update_age_spec.rb:15:in `block (3 levels) in <main>'
```

documentation/chat/frontend/scripts/prism-linenumbers.js

@@ -0,0 +1,57 @@
+(function() {
+
+if (typeof self === 'undefined' || !self.Prism || !self.document) {
+  return;
+}
+
+Prism.hooks.add('complete', function (env) {
+  if (!env.code) {
+    return;
+  }
+
+  // works only for <code> wrapped inside <pre> (not inline)
+  var pre = env.element.parentNode;
+  var clsReg = /\s*\bline-numbers\b\s*/;
+  if (
+    !pre || !/pre/i.test(pre.nodeName) ||
+      // Abort only if nor the <pre> nor the <code> have the class
+    (!clsReg.test(pre.className) && !clsReg.test(env.element.className))
+  ) {
+    return;
+  }
+
+  if (env.element.querySelector(".line-numbers-rows")) {
+    // Abort if line numbers already exists
+    return;
+  }
+
+  if (clsReg.test(env.element.className)) {
+    // Remove the class "line-numbers" from the <code>
+    env.element.className = env.element.className.replace(clsReg, '');
+  }
+  if (!clsReg.test(pre.className)) {
+    // Add the class "line-numbers" to the <pre>
+    pre.className += ' line-numbers';
+  }
+
+  var match = env.code.match(/\n(?!$)/g);
+  var linesNum = match ? match.length + 1 : 1;
+  var lineNumbersWrapper;
+
+  var lines = new Array(linesNum + 1);
+  lines = lines.join('<span></span>');
+
+  lineNumbersWrapper = document.createElement('span');
+  lineNumbersWrapper.setAttribute('aria-hidden', 'true');
+  lineNumbersWrapper.className = 'line-numbers-rows';
+  lineNumbersWrapper.innerHTML = lines;
+
+  if (pre.hasAttribute('data-start')) {
+    pre.style.counterReset = 'linenumber ' + (parseInt(pre.getAttribute('data-start'), 10) - 1);
+  }
+
+  env.element.appendChild(lineNumbersWrapper);
+
+});
+
+}());