Add a viable monochrome colorscheme

[neutaaaaan]

Here is a revised version of the catch-all, should work virtually everywhere, on virtually everything, monochrome colorscheme I made about a year ago.

It only uses the user's physical terminal or terminal emulator foreground and background colors, and looks exactly the same with termguicolors and the usual t_Co values, including 0.

Because of the low-bandwidth that only relying on bold, underline and reverse provides, the focus is on usability and avoiding visual noise unless absolutely necessary rather than being feature-complete. For example, diffs are perfectly possible, but DiffChange doesn't do anything: only the modifications themselves are highlighted. It's not ideal, but short of potentially underlining the entire screen, that's the compromise I settled for.

There are some slight ui inconsistencies for similar reasons: what's meant to always be displayed, such as a long TabLine, shouldn't blind the user. On the other hand, a menu is only going to be on screen for a very short while, and it's assumed that the user will want the entire thing to "pop" so they can pick the correct option and go back to what they were initially doing.

[habamax]

windows terminal looks weird:

[neutaaaaan]

I can't look into that as I don't have a windows machine handy. The problem didn't seem to happen last year, and I don't believe I've made any significant chances since besides refactoring it.

[habamax]

it works for everything else I tried it with though

[habamax]

Aha, so windows terminal kind of bleh with cleared Normal -- here is default:

[neutaaaaan]

Closing this to clean a few things up on my end and add Terminal, will pr again later.