Improve styling of the gutter and < >

[ahuang11]

1. Don't reserve width space for gutter to save on space and match expectations (similar to other apps); instead overlay < > on the left/right panels
2. Better alignment of < | >
3. Show thin blue line on drag

New HSplit:

hsplit.mp4

Old HSplit:

hsplit_old.mp4

New VSplit

vsplit.mp4

Old VSplit

vsplit_old.mp4

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.25%. Comparing base (493c628) to head (88685f9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #30   +/-   ##
=======================================
  Coverage   87.25%   87.25%           
=======================================
  Files           3        3           
  Lines         102      102           
=======================================
  Hits           89       89           
  Misses         13       13           

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