Include language-configuration.json in vsix (#383)

This was preventing commenting and indenting operations in VS Code from
working as expected when run from VSIX (as opposed to F5) since the file
was not actually put in the package.

packages/adl-vscode/.vscodeignore

@@ -9,5 +9,6 @@
 !dist/**/*.js
 !dist/**/*.js.map
 !extension-shim.js
+!language-configuration.json
 !README.md
 !ThirdPartyNotices.txt