This is a breaking change.

This updates `std.zig.fmtId` to support conditionally escaping
primitives and the reserved `_` identifier via format specifiers:

- `{}`: escape invalid identifiers, identifiers that shadow primitives
  and the reserved `_` identifier.
- `{p}`: same as `{}`, but don't escape identifiers that
  shadow primitives.
- `{_}`: same as `{}`, but don't escape the reserved `_` identifier.
- `{p_}` or `{_p}`: only escape invalid identifiers.

(The idea is that `p`/`_` mean "allow primitives/underscores".)

Any other format specifiers will result in compile errors.

Additionally, `isValidId` now considers `_` a valid identifier. If this
distinction is important, consider combining existing uses of this
function with the new `isUnderscore` function.