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/plugin/settings/lspconfig.lua

@@ -74,3 +74,12 @@ if utils.is_executable("rust-analyzer") then
         on_attach = lsp.on_attach,
     })
 end
+
+-- Shell
+if utils.is_executable("bash-language-server") then
+    lspconfig.bashls.setup({
+        filetypes = { "bash", "sh", "zsh" },
+        capabilities = capabilities,
+        on_attach = lsp.on_attach,
+    })
+end