feat(pack): add elm language pack (#653)

Co-authored-by: gacallea <[email protected]>

lua/astrocommunity/pack/elm/README.md

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+# Elm Language Pack
+
+This plugin pack does the following:
+
+- Adds `elm` Treesitter parsers
+- Adds `elm-language-server` language server
+- Adds `elm-format` formatter
+
+## LSP Configuration
+
+The default
+[elm-language-server](https://github.com/elm-tooling/elm-language-server)
+configuration matches all [currently supported features for Neovim
+LSP](https://github.com/elm-tooling/elm-language-server#editor-support).
+
+## A note on elm-test and elm-review
+
+`elm-test` is not directly supported by [neo-test](../test/neotest) but it's
+available via
+[neotest-vim-test](https://github.com/nvim-neotest/neotest-vim-test).
+
+However, it is common practice to peruse both `elm-test` and `elm-review`
+directly via the command line.

lua/astrocommunity/pack/elm/init.lua

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+local utils = require "astronvim.utils"
+
+return {
+  {
+    "nvim-treesitter/nvim-treesitter",
+    opts = function(_, opts)
+      if opts.ensure_installed ~= "all" then
+        opts.ensure_installed = utils.list_insert_unique(opts.ensure_installed, { "elm" })
+      end
+    end,
+  },
+
+  {
+    "williamboman/mason-lspconfig.nvim",
+    opts = function(_, opts) opts.ensure_installed = utils.list_insert_unique(opts.ensure_installed, { "elmls" }) end,
+  },
+
+  {
+    "jay-babu/mason-null-ls.nvim",
+    opts = function(_, opts) opts.ensure_installed = utils.list_insert_unique(opts.ensure_installed, { "elm_format" }) end,
+  },
+}