feat: support customize supported node types for slash menu

[LucasXu0]

Use supportSlashMenuNodeTypes to customize which nodes support the slash menu.

CharacterShortcutEvent customSlashCommand(
  List<SelectionMenuItem> items, {
  // ...
  Set<String> supportSlashMenuNodeTypes = _defaultSupportSlashMenuNodeTypes,
})

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.18%. Comparing base (ac3b090) to head (36496fa).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...ent/service/shortcuts/character/slash_command.dart	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #982      +/-   ##
==========================================
- Coverage   72.19%   72.18%   -0.02%     
==========================================
  Files         318      318              
  Lines       15147    15141       -6     
==========================================
- Hits        10936    10929       -7     
- Misses       4211     4212       +1     

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