sanitycheck: parse ecma2020 constructs (??)

[bobrippling]

To help with PRs such as #3642

[bobrippling]

@gfwilliams this PR assumes we can parse all syntaxes from the 2020 edition

BangleApps/bin/sanitycheck.js

Line 29 in a8f1aa9

return str => parser.parse(str, { ecmaVersion: 2020 });

... but I don't think that's the case (e.g. destructuring isn't supported), so I have two questions:

1. Is there a list anywhere of what's supported and what isn't?
2. Does this matter or do we just accept that we might pass some things through sanitycheck which then fail to parse on-device? Or vice-versa, we only enable ?? parsing and possibly other things espruino supports (based on the above list)

[gfwilliams]

Is there a list anywhere of what's supported and what isn't?

https://www.espruino.com/Features

I guess ideally it'd be really nice to throw errors for unsupported stuff. Is there a 'visitor' for acorn maybe, so we can iterate over everything in the syntax tree and check for unsupported stuff that gets parsed (eg spread I guess)?