Add MultiSplit and simplify code

[philippjfr]

• Adds new MultiSplit
• Removes Split.invert
• Makes Split.collapsed a tri-state (0, 1, None)
• Rename min_sizes -> min_size
• Exposes snap_size, max_size, step_size
• Changes button behavior to take into account current sizes, i.e. snap to expanded size or collapsed state depending on current size
• Abandon collapsed state events
• Simplify code
• Add UI tests

[codecov]

Codecov Report

❌ Patch coverage is 75.92593% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.91%. Comparing base (bc33a56) to head (158e0d2).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/panel_splitjs/base.py	75.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
+ Coverage   72.41%   73.91%   +1.49%     
==========================================
  Files           3        3              
  Lines          87       92       +5     
==========================================
+ Hits           63       68       +5     
  Misses         24       24              

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[philippjfr]

Will merge despite test failures, they seem to be related to a panel compile issue that will be fixed in panel 1.8.3.