feat: add air lsp (#3614)

* feat: add air lsp

* fix(air): use `vim.fs.root()`

Author: m-muecke

lua/lspconfig/configs/air.lua

@@ -0,0 +1,18 @@
+return {
+  default_config = {
+    cmd = { 'air', 'language-server' },
+    filetypes = { 'r' },
+    root_dir = vim.fs.root(0, { 'air.toml', '.air.toml', '.git' }),
+    single_file_support = true,
+  },
+  docs = {
+    description = [[
+https://github.com/posit-dev/air
+
+Air is an R formatter and language server, written in Rust.
+
+Refer to the [documentation](https://posit-dev.github.io/air/editors.html) for more details.
+
+  ]],
+  },
+}