Adapt to utf8 changes

JS_SetCStringsAreUTF8 has been removed and internally spidermonkey no longer handles
utf8 strings. There is a compiletime option setUTF8 that needs to be set, this change
requires using JS::Evaluate. Additionally we need to handle utf8 encoding in the
gjs_string functions. Finally JS_BufferIsCompilableUnit no longer has a utf8 argument

https://bugzilla.gnome.org/show_bug.cgi?id=711046