Problem: Not enough tests for the slice() function.
Solution: Test with multibyte chars, and in both Legacy and Vim9 script.
Update docs to be clearer about how it treats composing chars.
(zeertzjq)
closes: #14275
Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>