eglot-server-programs: Add support for nil

[leungbk]

• eglot.el (eglot-server-programs): Add nil

• README.md: Mention nil

• NEWS.md: Mention nil

This is more featureful and more actively maintained than rnix-lsp, so I have placed it first in eglot-alternatives.

[bhankas]

Another nil user here, can confirm it is much more responsive and almost useful compared to rnix-lsp which I never quite got to being useful in any nix buffer ever.

[leungbk]

Submitted the patch to mainline Emacs instead.

[joaotavora]

OK, no problem. Be sure to CC: me in these patches in the future so I don't miss them.