Add support for base16-vim alongside nvim-base16

[JamyGolden]

Currently lualine.nvim only supports nvim-base16, this adds support for base16-vim.

base16-vim and nvim-base16 both use the base16- prefix for colorscheme names:

• https://github.com/tinted-theming/base16-vim/tree/main/colors
• https://github.com/RRethy/nvim-base16/tree/master/colors

This causes colorschemes from both plugins to trigger the lua/lualine/themes/base16.lua file, which is why I made the changes in that file.

• Check if a base16-vim colorscheme is active (by testing to see if the first and last colour variables are set). If it is, use the existing global variables, if not then continue to load and setup nvim-base16.
• Update readme in context of changes

I can add some more context for the base16-vim and tinted-theming if necessary, just let me know.

[titaniumtraveler]

Seems my PR (#1081) is a duplicate of this.
The tests seem to be fixed now and a simple rebase should merge those changes and rerun the tests.
Though I'm not sure if the maintainer are going to merge the oldish pull-requests.

[JamyGolden]

Thanks for letting me know about updating the PR to run the tests. This PR has been here for a while without any engagement so I'm not sure if it will ever be merged. I really just want the functionality to be in lualine, so if you're able to get your PR merged, then go ahead! 😄

[titaniumtraveler]

I guess I will write an issue or something, in the mean time I just have been using a custom theme that basically does the same thing.

[JamyGolden]

@shadmansaleh any chance you can merge one of the PRs which add base16-vim support?

[shadmansaleh]

Thanks, @JamyGolden and @titaniumtraveler both of you for your contribution. Sorry, for responding so late. These PRs sort of got lost.

[titaniumtraveler]

@shadmansaleh No problem. I didn't really need to have that upstream, because I was essentially doing it as custom theme. But now I can just delete that code, which feels nice.
Also other people using lualine and base16-vim now just get builtin support, which is great!