Add context-aware autocomplete for QueryDescendants selectors

[JohnnyMorganz]

TODO: We now trigger a lot of completion for common punctuation. Is this a performance concern? Maybe we need to scope it down to just ClassNames.

A lot of roblox specific details are leaking into the generic Completion.cpp, can we improve this?

Summary

• Adds context-aware autocomplete inside Instance:QueryDescendants("...") selector strings
• Understands CSS-like selector syntax and provides appropriate completions based on cursor position
• Builds on the magic type narrowing added in Add magic type narrowing for Instance:QueryDescendants #1367

Completion Contexts

Cursor position	Completions
Empty string or after >, >>, ,, space	All Instance subclass names
After :	Pseudo-classes (not, has) with snippet parentheses
Inside [...]	Properties of the preceding class (or Instance if none)
After . or #	None (tags/names have no LSP data available)

Implementation Details

• RobloxLuauExt.cpp: Attaches "QuerySelector" tag to QueryDescendants
• RobloxCompletion.cpp: Selector context parser (detectSelectorContext, findPrecedingClassName) and "QuerySelector" handler in completionCallback. Extracts shared getPropertiesOfType() helper to deduplicate with existing "Properties" handler
• Completion.cpp: Computes string content up to cursor for context detection; token-level textEdit replaces only the current token instead of the full string; pseudo-class entries get snippet parentheses and documentation
• LanguageServer.cpp: Registers [, (, ,, > as completion trigger characters, suppressed outside QuerySelector context

Test plan

• 22 test cases covering all selector contexts, mid-identifier completion, textEdit ranges, pseudo-class snippets/documentation, fallback to Instance properties, and no-completion cases
• Full test suite passes (677 tests)
• Tests pass with --new-solver

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