Experimenting on at-mentions popover positioning

[acelaya]

POC on adding a Popover that is displayed when writing @ in the annotations textarea, and is positioned relative to the caret position.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 14 lines in your changes missing coverage. Please review.

Project coverage is 99.29%. Comparing base (c738173) to head (50c7cd0).

Files with missing lines	Patch %	Lines
src/sidebar/components/MarkdownEditor.tsx	60.00%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6705      +/-   ##
==========================================
- Coverage   99.43%   99.29%   -0.14%     
==========================================
  Files         271      271              
  Lines       10215    10250      +35     
  Branches     2434     2437       +3     
==========================================
+ Hits        10157    10178      +21     
- Misses         58       72      +14     

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[acelaya]

This has been a good experiment, but it's currently trying to solve two problems at once: 1) popover positioning based on caret location, and 2) when should the popover open or close.

In order to ease reviwwing, I'm going to close this PR and move the logic to three separate PRs:

1. Create a popover that anchors to the textarea itself and implement the open/close logic based on what is written.
2. Move the anchor to a "virtual" element that ensures the popover shows next to the caret location (this has been the main focus of this PR)
3. Display a list of users who already annotated the document, and allow client-side filtering and arrow-key navigation.

The three of them will be hidden behind a feature flag.