There are two paths for decoding UTF8 strings directly to JSString:
JS_NewStringCopyUTF8N() and JS_NewStringCopyUTF8Z(). Mostly we have a
zero-terminated string in GJS, so we shorten gjs_string_from_utf8() to
remove its string length argument and use JS_NewStringCopyUTF8Z().

There are a few places where we have a non-zero-terminated string, so we
introduce gjs_string_from_utf8_n() to handle that path.

A few places we were doing a UTF8 conversion where it was not necessary.