Mastodon counts post lengths by considering emojis to be single
characters, no matter how many unicode code points they are composed of.
So "😜" has length 1.
Pachli was using `String.length`, which considers "😜" as length 2.
Correct the calculation by using a BreakIterator to count the characters
in the string, which treats multi-character emojis as a length 1.
Poll options had a similar problem, exacerbated by the Mastodon web UI
also having the same problem, see
https://github.com/mastodon/mastodon/issues/28336.
Fix that by creating `MastodonLengthFilter`, an `InputFilter` that does
the right thing for regular text that may contain emojis.
See also https://github.com/tuskyapp/Tusky/pull/4152, which has the fix
for status length but not polls.
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Co-authored-by: Konrad Pozniak <opensource@connyduck.at>
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