feat: Ansi slice function

[sukus21]

Based on the truncate function.
Specify a start point (inclusive) and end point (exclusive) in cell-width units, and get a string back at that exact size.
Handles ansi-codes and multi-length characters.

[ccoVeille]

I like the code, the quality of comments, the extensive tests. 🥰

[ccoVeille]

I find the code clearer now, BTW.

Thanks for baring with my remarks

[aymanbagabas]

LGTM, thanks @sukus21 for your contribution!

[aymanbagabas]

@sukus21, I've reverted this PR. I think we need to think through the implication of adding space paddings to the sliced string. I'm not sure if that's the right approach here. Take string slicing for example, something like s[1:3] will slice the string as if it was a byte slice and breaking utf8 codepoints.
Using something like golang.org/x/exp/utf8string Slice accounts for runes and returns the sliced string with respect to utf8 codepoints (runes) and the start and end arguments resemble utf8 rune indices.

With that being said, I'm not sure how we should handle slicing wide characters and I don't think padding with spaces is the right solution, especially if end > StringWidth(s).

Could you send a new draft PR so we can discuss this more?