Support multiple forms of inline suppression

[fredemmott]

HHAST_FIXME and HHAST_IGNORE_ERROR were originally intended to mirror HH_FIXME vs HH_IGNORE_ERROR

• HH_IGNORE_ERROR has been removed
• the 'error' part makes some people feel more uncomfortable suppressing linters than they should; linters should generally be taken as 'this is fishy', not a hard error. There are valid reasons to e.g. await in a loop, and suppressing the linter is better/clearer than refactoring to avoid triggering it

Linters can provide their own override, but that replaces the default set.

I suggest:

1. changing this to a keyset<string> so that specific linters can add a supported suppression
2. adding a few more defaults:

• HHAST_IGNORE[Classname]: still consistent, but briefer, and doesn't have the negative connotations of FIXME or ERROR
• @lint-ignore ClassName: consistent with some other frameworks, and still a low chance of collision - and it doesn't matter if there is a collision

3. (optional) allow projects to configure their own additional suppression comments for each linter

• for example, could support @lint-ignore FacebookInternalLinterName in the HSL