Add parameterLabel modifier for argument labels in semantic tokens

[PhantomInTheWire]

Motivation: Argument labels are currently tokenized as function without any modifier, making them indistinguishable from function names. This prevents theme authors from styling them differently, particularly important for accessibility (e.g., themes for neurodivergent developers) and personal preference.

Changes:

• Added parameterLabel semantic token modifier to SemanticTokenModifiers extension
• Applied the modifier to argument labels in both the syntactic path (SemanticTokens.swift) and semantic path (SyntaxHighlightingTokenParser.swift) to ensure consistency.
• Updated the token legend to include the new modifier so editors can decode it.
• Updated all(?) relevant test expectations to include .parameterLabel for argument label tokens.

Fixes #1540

[ahoppen]

Nice. Looks good to me, I’d just like to define the modifier in swift-tools-protocols.

[PhantomInTheWire]

@ahoppen i have a open pr on swift-tools-protocols for this. we will have to merge that first. swiftlang/swift-tools-protocols#31 before this