Adding a `high-browser-compatibility` flag

[tmke8]

It would restrict you to features that are supported by all browsers.

We could pass the flag to the lexer which then checks in a hashmap whether a given command is in the list of problematic commands. (See a different plan below.)

What should be on the list:

For Safari

• \bar
• \hat
• \dot, \ddot, \dddot, \ddddot
• \check
• \grave
• \breve
• \acute
• \vec

For Chrome

• \widetilde{xxx} (no stretch)
• \widecheck{xxx} (no stretch)
• \overline{xxx} (no stretch)
• \underline{xxx} (no stretch)

Given that these all are related to accents, it might make more sense to pass the flag to the parser and have a field on the accent token for whether it's safe to use. Or: we do add a marker field on the accent token, but it's read by the lexer; the advantage of this is that we can report a better error.