Allow a non-ASCII character in the remaining identifier rules

`foo!`, `foo=`, `@foo` and `@@foo` still asked for `[a-zA-Z_]` at the
front. None of them cares about the case of that character, so
widening that position to `ident_start` is all they need to accept the
same names Ruby does.

`@rbs` keeps winning over `@` + identifier: the two match equally far
and the keyword is written first.

Author: soutaro