js: support UTF-8 mujs API with a header-only wrapper

The mujs C API expects input strings to be encoded as CESU-8, and
similarly strings from the VM are CESU-8 encoded.

CESU-8 differs from UTF-8 encoding only for codepoints >= U+10000 .

Till now mpv ignored this requirement and as a result emoji and other
non-BMP codepoints were not processed correctly (nothing blew though).

Now such codepoints work and converted correctly in all mujs APIs
to/from CESU-8, including in script source files.

This commit uses a single-header wrapper which replaces all the mujs
CESU-8 string APIs with UTF-8 ones with identical names.

In mpv it's almost entirely fully transparent because mpv currently
doesn't use custom allocator or callbacks (which are not wrapped
automatically).

mpv does use js_Report, but for now we'll live with the rare cases
where reports require conversion to UTF-8 (possibly reported function
names with emoji etc), though it's trivial to make it UTF-8 as well.

The wrapper header - mujsutf8.h - is stand alone and does not depend
on any mpv code or headers.

player/javascript.c

@@ -26,6 +26,7 @@
 #include <stdint.h>
 
 #include <mujs.h>
+#include "mujsutf8.h"
 
 #include "osdep/io.h"
 #include "mpv_talloc.h"