runfix: Handle mention deletion edge cases

[atomrc]

Description

fixes a bunch of edge case when dealing with deleting mentions, namely:

• deleting mention when the entire text is selected
• fixing the selection after a mention has been deleted

Screenshots/Screencast (for UI changes)

Screen.Recording.2023-11-08.at.15.46.18.mp4

Checklist

• PR has been self reviewed by the author;
• Hard-to-understand areas of the code have been commented;
• If it is a core feature, unit tests have been added;

[codecov]

Codecov Report

Merging #16167 (da75514) into dev (dfa6dd7) will decrease coverage by 0.02%.
Report is 1 commits behind head on dev.
The diff coverage is 4.76%.

@@            Coverage Diff             @@
##              dev   #16167      +/-   ##
==========================================
- Coverage   44.69%   44.68%   -0.02%     
==========================================
  Files         706      706              
  Lines       23100    23092       -8     
  Branches     5262     5266       +4     
==========================================
- Hits        10325    10318       -7     
  Misses      11444    11444              
+ Partials     1331     1330       -1