Add rnix-lsp for Nix diagnostics and completion

ale_linters/nix/rnix_lsp.vim

@@ -0,0 +1,16 @@
+" Author: jD91mZM2 <me@krake.one>
+" Description: rnix-lsp language client
+
+function! ale_linters#nix#rnix_lsp#GetProjectRoot(buffer) abort
+    " rnix-lsp does not yet use the project root, so getting it right is not
+    " important
+    return fnamemodify(a:buffer, ':h')
+endfunction
+
+call ale#linter#Define('nix', {
+\   'name': 'rnix_lsp',
+\   'lsp': 'stdio',
+\   'executable': 'rnix-lsp',
+\   'command': '%e',
+\   'project_root': function('ale_linters#nix#rnix_lsp#GetProjectRoot'),
+\})