initial syntax highlighting work

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I9469167a04228dc43f4b67d8bddc090a6a6a6964

ftplugin/direnv.lua

@@ -0,0 +1,47 @@
+if vim.b.did_ftplugin then
+   return
+end
+vim.b.did_ftplugin = true
+
+-- Set buffer-local commentstring
+vim.bo.commentstring = "# %s"
+
+-- Saving an .envrc (or related) should automatically trigger the plugin's
+-- reload/refresh behavior without causing errors when the plugin/command
+-- is not installed.
+local group = vim.api.nvim_create_augroup("DirenvBuffer", { clear = false })
+vim.api.nvim_create_autocmd("BufWritePost", {
+   group = group,
+   buffer = 0,
+   callback = function()
+      -- Trigger the Direnv reload command if available
+      if vim.fn.exists(":Direnv") == 2 then
+         pcall(vim.cmd, "Direnv reload")
+      end
+   end,
+})
+
+-- Decide whether to use tree-sitter or legacy syntax script. Tree-sitter
+-- is more modern and is almost always preferrable to Regex-based highlighting
+-- that the legacy method requires, but it's difficult to bundle a Tree-sitter
+-- grammar, so we cannot unconditionally load my own Direnv grammar. Though, if
+-- it IS available then we should skip legacy syntax highlighting.
+local has_ts = false
+local ok, parsers = pcall(require, "nvim-treesitter.parsers")
+if ok and type(parsers.has_parser) == "function" then
+   has_ts = parsers.has_parser("direnv")
+end
+
+if has_ts then
+   -- If a parser exists, prefer tree-sitter highlighting. Nothing to do
+   -- here because nvim-treesitter will attach based on filetype by itself.
+   return
+end
+
+-- Set syntax to direnv so Vim will load the bundled syntax file. Only set
+-- if no syntax is already active for this buffer, i.e., if tree-sitter is
+-- not loaded/available.
+if not vim.bo.syntax or vim.bo.syntax == "" then
+   -- setlocal syntax=direnv should cause Vim to load syntax/direnv.vim from runtime
+   pcall(vim.cmd, "setlocal syntax=direnv")
+end