utility/preview: added markdown-previewer.nvim and moved glow

[Donnerinoern]

Description

Added markdown-previewer.nvim as a Markdown previewer and moved both it and glow to a new category: utility/preview.

Type of change

• New feature
• Breaking change (move of glow may break some configs)

Checklist

• My code follows the style and contributing guidelines of this project.
• I ran Alejandra to format my code (nix fmt).
• I have performed a self-review of my own code and tested it.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.
• This change requires a documentation update.
• I have updated the documentation accordingly.

Screenshots & Logs

[NotAShelf]

I'm not sure if we really want the previewer in languages/markdown.nix directly. Generally companion plugins are sorted by function and not language, going under !utilty for the most part. This is fine but I'd rather avoid non-LSP plugins in the language modules.

[Donnerinoern]

Fair point. I thought about moving it, but didn't because Glow already was there. I'll keep that in mind for future PRs.

I'll make a new commit in a little bit.

[Donnerinoern]

I hope this new commit is satisfactory. If not, let me know, and I'll change it. It moves both glow and markdown-preview to a new preview category in utility. I also fixed types.listOf types.strand mkOption.

[Donnerinoern]

While the IP-address and port options do play nicely with empty strings, they're not specified to be empty strings in upstream, just empty (unlike other options). I therefore added optionalString just in case.

[NotAShelf]

LGTM!

requesting a review from @FrothyMarrow, given the relatively large scope of the PR in case I've missed anything

[FrothyMarrow]

👍

[NotAShelf]

LGTM!

[NotAShelf]

Merging when the CI passes.

[NotAShelf]

Missing documentation, but I'll add it. Thanks!