Enable JavaScript specific warning behavior

[billti]

See #6658 for background.

Currently for JavaScript files we provide a very limited set of errors, mostly for syntax errors. There are a number of grammatical and type errors that would be useful to display also.

Brief notes from ad hoc discussion in the team room include:

• Move the code that flags TypeScript syntax in JavaScript files for better error handling
• Add a distinction between errors and warnings to avoid "valid" JavaScript containing errors
• Make the warnings user configurable as part of the project (i.e. may enable/disable certain warnings)

More detailed design to come...

[egamma]

From our own usage for Salsa for editing JS, we miss a warning about undefined variables. In eslint:

{
    "rules": {
        "no-undef": 2,
        "no-unused-vars": 1
    }
}

The other warning we got used is the one about unused local variables.