Implement multi-file doc highlights

[jakebailey]

Uses the same kind of idea as #3311, except that this is a separate method in VS Code, so it is in our LSP extension.

[Copilot]

Pull request overview

Adds a custom LSP request + VS Code proposed API integration to support document highlights across multiple files (multi-document highlights), along with updated/added fourslash tests and baselines to validate the new behavior.

Changes:

• Introduces custom/textDocument/multiDocumentHighlight in lsproto + internal/lsp/server.go and wires it to a new LanguageService.ProvideMultiDocumentHighlights implementation.
• Updates fourslash infrastructure to request multi-document highlights with filesToSearch, and adds/updates generated tests + baselines.
• Implements a VS Code extension feature using the proposed MultiDocumentHighlightProvider API and bumps VS Code/@types versions to a proposal-supporting engine level.

Reviewed changes

Copilot reviewed 26 out of 28 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/ls/documenthighlights.go	Implements multi-file highlight computation and response shaping.
internal/lsp/server.go	Registers the custom LSP method handler and advertises capability.
internal/lsp/lsproto/_generate/generate.mts	Extends protocol model with request/structures + capability flag.
internal/lsp/lsproto/lsp_generated.go	Generated protocol types for the new request/response.
_extension/src/vscode.proposed.multiDocumentHighlightProvider.d.ts	Adds proposed VS Code API typings used by the extension.
_extension/src/languageFeatures/documentHighlight.ts	Client-side provider that calls the custom LSP method and converts results.
_extension/src/client.ts	Registers the new feature with the language client.
_extension/package.json	Enables the proposal + updates VS Code engine requirement.
internal/fourslash/fourslash.go	Adds VerifyBaselineDocumentHighlightsWithOptions (filesToSearch) and custom request path.
internal/fourslash/_scripts/convertFourslash.mts	Updates conversion to support multi-file document highlight baselines/options.
internal/fourslash/tests/gen/*.go	Updates existing tests and adds new generated tests covering multi-file behavior.
testdata/baselines/reference/fourslash/documentHighlights/*.jsonc	Baseline updates/new baselines including the filesToSearch header.
package-lock.json	Updates @types/vscode and engine metadata.