Adds four new  modifiers that can be used in themes:

- undercurled
- underdashed
- underdotted
- double-underline

Add underline field to doctests, and fix bugs

The cxterminfo crate has been used over popular alternatives
like `term` since it supports querying for extended capabilities
and also for it's small codebase size (which will make it easy
to inline it into helix in the future if required).

Underline styles are mutally exclusive and overwrite each other.
Therefore implementing as an modifier lead to incorrect behaviour
when the underline style is overwritten.

For backwards compatability the "underline" modified is retained (but
deprecated). Instead the "underline_style" and "underline_color"
optios should be used to style underlines.

…erline colors

Co-authored-by: Omnikar <[email protected]>