patch 9.1.0199: Not enough tests for the slice() function

patch 9.1.0199: Not enough tests for the slice() function

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>