Define colors for the integrated terminal

[rvighne]

This sets the 16 default colors within the Vim 8 integrated terminal (via g:terminal_ansi_colors) so that terminal apps look as expected. The colors are in terms of the already-defined variables, so it should stay consistent and work in light mode too.

Here's a comparison with One Half Dark from Windows Terminal. Note that WT decided to use a very bright color for white, but I've chosen to stick with the colorscheme so it looks better next to actual editor buffers.

[lygamac]

Terminal color was removed on #104 because the colors were inconsistent when using a light theme.

I second to re-add the colors, provided that we add support to the light theme too.

[rvighne]

These colors use the variables defined by the theme, like syntax_bg/syntax_fg. Since those are set conditionally based on the light/dark mode, I believe it works fine in that case. Here is a screenshot:

Other terminal apps also look OK (actually I'm not at all a fan but actual light mode users can judge whether this looks correct 😁):

[rvighne]

Due to inactivity from the maintainers, I've merged this (and some other long-standing PR's) into my fork: https://github.com/rvighne/vim-one/pull/3