This patch makes neovim look for tree-sitter parsers in the directories
specified by TREE_SITTER_GRAMMAR_PATH.  Thereby, making it compatible
with the tree-sitter setup used by other Guix packages. e.g. emacs.

diff --git a/runtime/lua/vim/treesitter/language.lua b/runtime/lua/vim/treesitter/language.lua
index f70f99421c..480e7bb77a 100644
--- a/runtime/lua/vim/treesitter/language.lua
+++ b/runtime/lua/vim/treesitter/language.lua
@@ -126,12 +126,19 @@ function M.add(lang, opts)
       return nil, string.format('Invalid language name "%s"', lang)
     end
 
-    local fname = 'parser/' .. lang .. '.*'
-    local paths = api.nvim_get_runtime_file(fname, false)
-    if #paths == 0 then
-      return nil, string.format('No parser for language "%s"', lang)
+    local paths = vim.split(os.getenv('TREE_SITTER_GRAMMAR_PATH') or '', ':')
+    for _, elem in ipairs(paths) do
+      local fname = elem .. '/' .. 'libtree-sitter-' .. lang .. '*'
+      local paths = vim.fn.glob(fname, false, true)
+      if #paths > 0 then
+        path = paths[1]
+        break
+      end
     end
-    path = paths[1]
+  end
+
+  if path == nil then
+    return nil, string.format('No parser for language "%s"', lang)
   end
 
   local res = loadparser(path, lang, symbol_name)