feat: add undercurl support

[strongholder]

Context

Modern development environments, especially those utilizing Neovim or Helix, frequently rely on the Language Server Protocol (LSP) to use the undercurl style for feedback. This underlining style, prevalent in many spell checkers over time, serves as an intuitive indicator for mistakes and errors. Recognizing its broad acceptance and utility, I am convinced of the importance of undercurl support for Zellij. The absence of this feature could significantly impact my continued engagement with the project.

Scope and Implementation

While I recognize the diverse underline styles supported by various terminal emulators (including double, curly, dotted, and dashed), the prominence and functionality of undercurl make it a priority. Hence, this PR exclusively addresses the implementation of undercurl.

Related Issues

#1594
#2287

Tests

I've tested this implementation manually with Kitty and Alacritty

Kitty NeoVim:

Alacritty NeoVim:

Kitty Terminal:

Alacritty Terminal:

[crabdancing]

Awesome! Finally :D

[eatgrass]

Is it possible to implement along with all these styles as well?

vim/vim#9553

[strongholder]

@eatgrass That would be nice, however there's already a PR aiming to implement multiple underline styles, unfortunately it's still not merged, so I was hoping that we could at least get undercurl. Here's the PR in question - #2730. I really hope that the maintainers consider merging any of these PRs because it seems that a lot of people are interested in this feature.

[eatgrass]

@strongholder, thanks to you.
I was trying to add those styles based on your commit first; however, I found it a little tricky to manage underline rendering with multiple states, so I opened #2730. I'm sorry I didn't provide further notification here at that time.
I'm also looking forward to the maintainers merging these features.