Add overlays

[a-kenji]

Summary & Motivation

Add overlay to outputs of the flake. That way people can compose the unreleased versions better.

Add rnix-lsp overlays to the flake outputs, in order to be directly consumed by other flakes.

Provides 2 overlays: - default: with package named rnix-lsp, so people can overwrite their existing configuration - nightly: with package named rnix-lsp-nightly, so people can opt in to this package on a case-by-case basis

Also for now still uses the overlay attribute, for backwards compatibility, is that needed?

fixes: #77