Enable proposed API.

Signed-off-by: Yaohai Zheng <yaozheng@microsoft.com>

Author: yaohaizh

package-lock.json

@@ -9,6 +9,7 @@
   "publisher": "redhat",
   "bugs": "https://github.com/redhat-developer/vscode-java/issues",
   "preview": true,
+  "enableProposedApi": true,
   "engines": {
     "vscode": "^1.31.0"
   },