home: vim: lspconfig: migrate to 'bashls'

Since `none-ls` has removed their `shellcheck` built-in. This actually
makes the diagnostics more robust to POSIX/non-POSIX scripts (the LSP
server detects it at runtime, which is more robust than the `ftdetect`
scripts).

Nice bonus: the shellcheck code is shown in the diagnostics message
without any configuration!

I'm not sure if I can configure `avoid-nullary-conditions` -- though it
seems like this check is broken at the moment (I couldn't get it to
trigger during my tests).

modules/home/vim/default.nix

@@ -68,7 +68,6 @@ in
       lsp-format-nvim # Simplified formatting configuration
       lsp_lines-nvim # Show diagnostics *over* regions
       none-ls-nvim # LSP integration for linters and formatters
-      none-ls-shellcheck-nvim # shellcheck shim for none-ls
       nvim-treesitter.withAllGrammars # Better highlighting
       nvim-treesitter-textobjects # More textobjects
       nvim-ts-context-commentstring # Comment string in nested language blocks
@@ -106,7 +105,7 @@ in
       nixpkgs-fmt
 
       # Shell
-      shellcheck
+      nodePackages.bash-language-server
       shfmt
     ];
   };