Polyfill utf8.char

[Validark]

No description provided.

[Validark]

It's pretty easy to verify this is the correct implementation btw. Just run this for all possible inputs in Lua 5.4 (and make it so utf8_char is always the custom version):

for i = 0, 1114111 do
    assert(utf8_char(i) == utf8.char(i))
end