DEV: Compile theme migrations javascript files when running theme qunit (#25219)

Why this change?

Currently, is it hard to iteratively write a theme settings migrations
because our theme migrations system does not rollback. Therefore, we
want to allow theme developers to be able to write QUnit tests for their
theme migrations files enabling them to iteratively write their theme
migrations.

What does this change do?

1. Update `Theme#baked_js_tests_with_digest` to include all `ThemeField`
records of `ThemeField#target` equal to `migrations`. Note that we do
not include the `settings` and `themePrefix` variables for migration files.

2. Don't minify JavaScript test files becasue it makes debugging in
   development hard.

app/models/theme.rb

@@ -913,21 +913,27 @@ class Theme < ActiveRecord::Base
 
   def baked_js_tests_with_digest
     tests_tree =
-      theme_fields
-        .where(target_id: Theme.targets[:tests_js])
-        .order(name: :asc)
-        .pluck(:name, :value)
-        .to_h
+      theme_fields_to_tree(
+        theme_fields.where(target_id: Theme.targets[:tests_js]).order(name: :asc),
+      )
 
     return nil, nil if tests_tree.blank?
 
-    compiler = ThemeJavascriptCompiler.new(id, name)
-    compiler.append_tree(tests_tree, for_tests: true)
+    migrations_tree =
+      theme_fields_to_tree(
+        theme_fields.where(target_id: Theme.targets[:migrations]).order(name: :asc),
+      )
+
+    compiler = ThemeJavascriptCompiler.new(id, name, minify: false)
+    compiler.append_tree(migrations_tree, include_variables: false)
+    compiler.append_tree(tests_tree)
+
     compiler.append_raw_script "test_setup.js", <<~JS
       (function() {
         require("discourse/lib/theme-settings-store").registerSettings(#{self.id}, #{cached_default_settings.to_json}, { force: true });
       })();
     JS
+
     content = compiler.content
 
     if compiler.source_map
@@ -942,6 +948,13 @@ class Theme < ActiveRecord::Base
 
   attr_accessor :theme_setting_requests_refresh
 
+  def theme_fields_to_tree(theme_fields_scope)
+    theme_fields_scope.reduce({}) do |tree, theme_field|
+      tree[theme_field.file_path] = theme_field.value
+      tree
+    end
+  end
+
   def to_scss_variable(name, value)
     escaped = SassC::Script::Value::String.quote(value.to_s, sass: true)
     "$#{name}: unquote(#{escaped});"