add utf16 code points range

[patricklx]

content-tag is counting chars. js length is counting utf16 CodePoints.

so, we want
str.encode_utf16().count()

much faster:
(if i did it right?)
perf

I think its more user friendly to let the user do str.slice(...), instead of [...str].slice(...).join('')#

ruste example

js example

any better naming suggestions?