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.json

@@ -296,6 +296,12 @@
             "%configuration.markdown.links.openLocation.beside%"
           ]
         },
+        "markdown.suggest.paths.enabled": {
+          "type": "boolean",
+          "default": true,
+          "description": "%configuration.markdown.suggest.paths.enabled.description%",
+          "scope": "resource"
+        },
         "markdown.trace": {
           "type": "string",
           "enum": [