Handle LSP textDocument/documentSymbol requests

[ahejlsberg]

This PR implements textDocument/documentSymbol request handling. This enables the following functionality in VS Code:

• Go to Symbol in Editor (Ctrl-Shift-O).
• Outline View.
• Breadcrumbs navigation bar.

Fixes #1295

[Copilot]

Pull Request Overview

This PR adds support for handling textDocument/documentSymbol requests in the LSP server, enabling symbol outline features in editors.

• Introduces a handleDocumentSymbol handler in the server.
• Implements ProvideDocumentSymbols and related helpers to traverse the AST and produce DocumentSymbol entries.
• Cleans up unused assignment-declaration kinds in the type checker.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/lsp/server.go	Register and implement textDocument/documentSymbol handling
internal/ls/symbols.go	Add ProvideDocumentSymbols and helper functions for AST traversal and symbol creation
internal/checker/types.go	Remove AssignmentDeclarationKind type and associated constants

Comments suppressed due to low confidence (3)

internal/lsp/server.go:799

• [nitpick] The variable name hover is misleading since it holds document symbols. Consider renaming it to symbols or docSymbols for clarity.

	hover, err := languageService.ProvideDocumentSymbols(ctx, params.TextDocument.Uri)

internal/ls/symbols.go:20

• No tests were added to validate ProvideDocumentSymbols. Add compiler tests in testdata/tests/cases/compiler to cover documentSymbol functionality and ensure correct output baselines.

func (l *LanguageService) ProvideDocumentSymbols(ctx context.Context, documentURI lsproto.DocumentUri) ([]*lsproto.DocumentSymbol, error) {

internal/checker/types.go:297

• You removed AssignmentDeclarationKind and its constants, which may break code relying on that type. Verify that all references are updated or confirm this deletion is intended.

	AccessFlagsPersistent                             = AccessFlagsIncludeUndefined