Add basic markdown link completions

For #140602

Only normal links for now. Will add reference links later. Should support the forms:

- `[](dir/file.md)`
- `[](./dir/file.md)`
- `[](/root-dir/file.md)`
- `[](#header)`
- `[](./dir/file.md#header)`

extensions/markdown-language-features/src/extension.ts

@@ -9,6 +9,7 @@ import * as commands from './commands/index';
 import LinkProvider from './features/documentLinkProvider';
 import MDDocumentSymbolProvider from './features/documentSymbolProvider';
 import MarkdownFoldingProvider from './features/foldingProvider';
+import { PathCompletionProvider } from './features/pathCompletions';
 import { MarkdownContentProvider } from './features/previewContentProvider';
 import { MarkdownPreviewManager } from './features/previewManager';
 import MarkdownSmartSelect from './features/smartSelect';
@@ -57,7 +58,8 @@ function registerMarkdownLanguageFeatures(
 		vscode.languages.registerDocumentLinkProvider(selector, new LinkProvider()),
 		vscode.languages.registerFoldingRangeProvider(selector, new MarkdownFoldingProvider(engine)),
 		vscode.languages.registerSelectionRangeProvider(selector, new MarkdownSmartSelect(engine)),
-		vscode.languages.registerWorkspaceSymbolProvider(new MarkdownWorkspaceSymbolProvider(symbolProvider))
+		vscode.languages.registerWorkspaceSymbolProvider(new MarkdownWorkspaceSymbolProvider(symbolProvider)),
+		PathCompletionProvider.register(selector, engine),
 	);
 }