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/package.nls.json

@@ -26,5 +26,6 @@
 	"configuration.markdown.links.openLocation.description": "Controls where links in Markdown files should be opened.",
 	"configuration.markdown.links.openLocation.currentGroup": "Open links in the active editor group.",
 	"configuration.markdown.links.openLocation.beside": "Open links beside the active editor.",
+	"configuration.markdown.suggest.paths.enabled.description": "Enable/disable path suggestions for markdown links",
 	"workspaceTrust": "Required for loading styles configured in the workspace."
 }