languages: Fix broken whitespace for C++

[phdavis1027]

Uses tree-sitter indent queries to create a pseudo-scope, since access modifiers are single-node in thetree-sitter grammar.

The @outdent capture truncates the parent { } indent range at the access specifier, keeping it at the same level as the class keyword and the pseudo-scopes from stacking.

The @outdent range selects siblings so we don't outdent the access specifiers themselves, since screenshots from the reference issue seem to show that at least one interested party does not prefer outdented access modifiers, and also it seemed like less of a departure from existing design.

Also, there are probably more tests than needed because this is my first time writing treesitter queries and I wanted to triple-guess myself. Quite open too feedback on which ones are more valuable.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Closes #53175

Release Notes:

• Improves handling of indentation after C++ access modifiers