feat(pack): Add nim pack (#699)

* feat(pack): Add nim pack

* feat(nim): Add formatter

lua/astrocommunity/pack/nim/README.MD

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+# Nim Language pack
+
+This plugin pack does the following:
+
+- Adds `nim` and `nim_format_string` Treesitter parsers
+- Adds `nim_langserver` language server
+- Adds `nimpretty` formatter'

lua/astrocommunity/pack/nim/init.lua

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