(v2) feat: add underline style and color

[aymanbagabas]

This adds support for setting the underline style and color. The underline style can be set to single, double, curly, dotted, or dashed. The color of the underline can be set to any color.

See https://sw.kovidgoyal.net/kitty/underlines/

Fixes: #488

[meowgorithm]

This looks good, though we may want to switch the adjective-noun order on the costs for better completions with LSPs:

// Before
NoUnderline
SingleUnderline
CurlyUnderline

// After
UnderlineNone
UnderlineSingle
UnderlineCurly

On the other hand, we already have NoColor, which works better with NoUnderline versus UnderlineNone, so maybe this is fine.

[meowgorithm]

Let's make the following API change:

// Before
NoUnderline
SingleUnderline
CurlyUnderline

// After
UnderlineNone
UnderlineSingle
UnderlineCurly

And also put caveats in the GoDocs to help users understand cases where this is and isn't supported.

Otherwise, this is good.

[meowgorithm]

Also a question: how does this behave in unsupported terminals? Will it fall back to having a normal underline? If not, can we make it fall back accordingly?

[aymanbagabas]

Also a question: how does this behave in unsupported terminals? Will it fall back to having a normal underline? If not, can we make it fall back accordingly?

It depends on the terminal and how they interpret the sequence. Apple Terminal for example, behaves incorrectly and doesn't support neither underline styles nor underline colors.

[meowgorithm]

@aymanbagabas in that case what happens? Does it underline at all?

[aymanbagabas]

@aymanbagabas in that case what happens? Does it underline at all?

No, it doesn't underline and instead writes the text with a background color.