feat(gsoc'24): markdown syntax extension for user tagging, circuit and video embedding

[Waishnav]

Fixes #15

Note: Please ignore the js build files changes. since they are needed to push for the engine to work in production

This PR's covers following functionality

• added the markdown syntax extension at both client side and server side for

1. Embedding circuits
2. Embedding video
3. Tagging user

• Feature flags are used for purpose of enabling and disabling this markdown extension.

SimpleDiscussion.setup do |config|
  config.markdown_circuit_embed = false # Default: true
  config.markdown_user_tagging = false # Default: true
  config.markdown_video_embed = false # Default: true
end

Screenshots:

1. Embed circuit modal when user clicks on embed-circuit button from toolbar
   

2. Embed video when user clicks on embed-video button from toolbar
   

3. User tagging dropdown
   

4. Server rendered forum post