This is a quick way to implement unresolved reference diagnostics.
For example, adding to VS Code config
"editor.tokenColorCustomizationsExperimental": {
"unresolvedReference": "#FF0000"
},
will highlight all unresolved refs in red.
In textmate, keyword.control is used for all kinds of things; in fact,
the default scope mapping for keyword is keyword.control!
So let's add a less ambiguous controlFlow modifier
See Microsoft/vscode#94367
