Don't use 2 characters for surrogates in identifiers because there is
support for the \u{} syntax when escaped identifiers were introduced.
Also catch eof errors while reading identifier names
Introduce ascii_identifiers:
By setting ascii_identifiers to undefined (default value),
ascii_identifiers will print identifiers using the same setting as
ascii_only within the limits of the ecmascript 6 grammar.
ascii_identifiers accept true and false, allowing identifiers to be
printed under different settings than strings with the ascii_only setting.
