fix: Adding back enter key extension with mentions

[Palanikannan1437]

Description

Added back the enter key extension at #2530, but this time working with Mentions

1. Comment Editor's Features
   The enter key is now remapped to submit the comment, while the 'shift-enter' key is mapped to the default enter key behavior. The edge cases for the various data structures/scenarios present in the comment editor have been handled as follows:

   1. Code Blocks: When the cursor is inside a code block, "Shift-Enter" inserts a newline character at the current cursor position, creating a new line within the code block.
   2. List Items: When the cursor is inside a list item, "Shift-Enter" will split the list item at the current cursor position, creating a new list item below the current one.
      End of Blocks: When the cursor is at the end of a block, "Shift-Enter" creates a new paragraph below the current block.
   3. Empty Blocks: When the cursor is inside an empty block that can be lifted, pressing "Shift-Enter" will lift the block out of its parent node.
   4. General Case: In all other cases, pressing "Shift-Enter" will split the block at the current cursor position, creating a new block below the current one.

   Here's a sleek demo video showcasing these changes (note I'm using Shift + Enter here as Enter key defaults to Submitting the comment itself)

Export-1698477557840.mp4

2. Fixed mentions to play along with Comment editor's enter extension
   The enter extension works properly now with mentions as we're keeping track of whether the mentions menu is open or not via a custom storage object in the extension and then using that modify the enter key extensions's behaviour.

Screen.Recording.2024-01-31.at.6.27.22.PM.mov