FEATURE: align with /filter and allow multiple category search (#27440)

This introduces the syntax of

`category:a,b,c` which will search across multiple categories.

Previously there was no way to allow search across a wide selection of
categories.

spec/lib/search_spec.rb

@@ -1255,6 +1255,41 @@ RSpec.describe Search do
       )
     end
 
+    it "allow searching for multiple categories" do
+      category2 = Fabricate(:category, name: "abc")
+      topic2 = Fabricate(:topic, category: category2)
+      post2 = Fabricate(:post, topic: topic2, raw: "snow monkey")
+
+      category3 = Fabricate(:category, name: "def")
+      topic3 = Fabricate(:topic, category: category3)
+      post3 = Fabricate(:post, topic: topic3, raw: "snow monkey")
+
+      search = Search.execute("monkey category:abc,def")
+      expect(search.posts.map(&:id)).to contain_exactly(post2.id, post3.id)
+
+      search = Search.execute("monkey categories:abc,def")
+      expect(search.posts.map(&:id)).to contain_exactly(post2.id, post3.id)
+
+      search = Search.execute("monkey categories:xxxxx,=abc,=def")
+      expect(search.posts.map(&:id)).to contain_exactly(post2.id, post3.id)
+
+      search = Search.execute("snow category:abc,#{category.id}")
+      expect(search.posts.map(&:id)).to contain_exactly(post.id, post2.id)
+
+      child_category = Fabricate(:category, parent_category: category2)
+      child_topic = Fabricate(:topic, category: child_category)
+      child_post = Fabricate(:post, topic: child_topic, raw: "snow monkey")
+
+      search = Search.execute("monkey category:zzz,nnn,=abc,mmm")
+      expect(search.posts.map(&:id)).to contain_exactly(post2.id)
+
+      search =
+        Search.execute(
+          "monkey category:0007847874874874874748749398384398439843984938439843948394834984934839483984983498394834983498349834983,zzz,nnn,abc,mmm",
+        )
+      expect(search.posts.map(&:id)).to contain_exactly(post2.id, child_post.id)
+    end
+
     it "should return the right categories" do
       search = Search.execute("monkey")