Cache name table on SourceFile

[jakebailey]

Same as Strada:

export function getNameTable(sourceFile: SourceFile): Map<__String, number> {
    if (!sourceFile.nameTable) {
        initializeNameTable(sourceFile);
    }

    return sourceFile.nameTable!; // TODO: GH#18217
}

function initializeNameTable(sourceFile: SourceFile): void {
    const nameTable = sourceFile.nameTable = new Map();
    sourceFile.forEachChild(function walk(node) {

[Copilot]

Pull request overview

This PR implements caching of the name table on SourceFile to avoid repeated computation, following the same pattern used in the TypeScript reference implementation. The name table is lazily initialized using sync.Once for thread-safe caching.

Key changes:

• Added cached name table with thread-safe lazy initialization to SourceFile struct
• Moved name table generation logic from a standalone function in utilities.go to a method on SourceFile in ast.go
• Updated call sites to use the new GetNameTable() method

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
internal/ast/ast.go	Added nameTableOnce and nameTable fields to SourceFile struct; implemented GetNameTable() method with lazy initialization using sync.Once
internal/ls/utilities.go	Removed getNameTable(), literalIsName(), and isTagName() functions that are now encapsulated within the GetNameTable() method
internal/ls/findallreferences.go	Updated call from getNameTable(sourceFile) to sourceFile.GetNameTable()
internal/ls/completions.go	Updated call from getNameTable(file) to file.GetNameTable()