C++ defines a number of keywords that are regular identifiers in
JavaScript, e.g. concept:
const concept = 1; // legit JS
This change expands the existing IsJavaScriptIdentifier(Tok) function
to return false for C++ keywords that aren't keywords in JS.
nit: may be worth adding a comment linking to:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#Keywords