[XELP] add custom support for @stop and @{} (ReplacementIdentifier)

[ljunkie]

support @Stop and @{} resource replacements.

• @Stop: treat the same as stop
• @{}: treat as an identifier (variable) with a specific type ReplacementIdentifier

[Copilot]

Pull request overview

This PR adds custom support for XELP resource replacement syntax in BrighterScript, introducing two new features: @stop as an alternative to the stop statement, and @{...} as replacement identifiers for resource placeholders. The implementation spans the lexer (tokenization), parser (AST generation), and type system (type assignment).

• Adds lexer support for recognizing @stop and @{...} syntax patterns
• Extends parser to handle @stop as a stop statement and @{...} as literal expressions
• Assigns DynamicType to replacement identifiers since their actual types are unknown until resource replacement

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/lexer/Lexer.ts	Adds tokenization logic for @stop (lines 165-170) and @{...} replacement identifiers (lines 163-164, 654-676)
src/lexer/TokenKind.ts	Defines new token kinds: ReplacementIdentifier and AtStop
src/parser/Parser.ts	Extends stop statement check to include AtStop, adds ReplacementIdentifier to primary expressions, fixes null check ordering (bug fix)
src/parser/tests/statement/Stop.spec.ts	Adds tests for @stop and @STOP lexing and parsing
src/parser/tests/ReplacementIdentifier.spec.ts	Comprehensive tests for @{...} syntax including nested identifiers and error cases
src/DiagnosticMessages.ts	Adds error message for unterminated replacement identifiers
src/util.ts	Maps ReplacementIdentifier tokens to DynamicType