Similar to another problem we had in CharacterData, we were assuming
that the offsets were raw utf8 byte offsets into the data, instead of
utf16 code units. Fix this by using the substring helpers in
CharacterData to get the text data from the Range.
There are more instances of this issue around the place that we will
need to track down and add tests for, but this fixes one of them :^)
For the test included in this commit, we were previously returning:
llo💨😮
Instead of the expected:
llo💨😮 Wo
We were previously assuming that the input offsets and lengths were all
in raw byte offsets into a UTF-8 string. While internally our String
representation may be in UTF-8 from the external world it is seen as
UTF-16, with code unit offsets passed through, and used as the returned
length.
Beforehand, the included test included in this commit would crash
ladybird (and otherwise return wrong values).
The implementation here is very inefficient, I am sure there is a
much smarter way to write it so that we would not need a conversion
from UTF-8 to a UTF-16 string (and then back again).
Fixes: #20971
This is a pretty straightforward test, but I managed to make this crash
on real sites while trying to fix#20971 without any other test in the
existing suite failing.
