We have gjs_string_to_utf8() which takes a JS::Value, checks if it holds
a string and throws an exception if not, then converts to a JSString and
calls JS_EncodeStringToUTF8() on it. This fixes some inefficient uses of
that function.

In a few cases the typecheck was repeated before calling
gjs_string_to_utf8(), the outer typecheck is simply removed.

In other cases the typecheck was already guaranteed because we wanted to
take a different code path when the value didn't hold a string, in these
cases we inline the value.toString() call and the JS_EncodeStringToUTF8()
call.

In many other cases we were converting a JSString to JS::Value only to
pass it to gjs_string_to_utf8() so we could convert it back to JSString.
In these cases we just use JS_EncodeStringToUTF8() directly.

gjs_string_to_utf8() is still useful when looking up a property on an
object that is supposed to be a string, and it's still used quite often
in the codebase.