gnu: neovim: honor TREE_SITTER_GRAMMAR_PATH.

While at it, also depend on tree-sitters that are builtin into neovim.

Fixes #2269

* packages/patches/neovim-tree-sitter-grammar-path.patch: New patch.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/vim.scm (neovim): Support TREE_SITTER_GRAMMAR_PATH.
[source] <patches>: Add patch.
[native-search-paths]: Add TREE_SITTER_GRAMMAR_PATH.
[propagated-inputs]: Add strictly required tree-sitter parsers.

gnu/local.mk

@@ -1978,6 +1978,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/nheko-0-12-1-fix-rendering-replies.patch	\
   %D%/packages/patches/nix-dont-build-html-doc.diff		\
   %D%/packages/patches/nfs4-acl-tools-0.3.7-fixpaths.patch	\
+  %D%/packages/patches/neovim-tree-sitter-grammar-path.patch	\
   %D%/packages/patches/network-manager-plugin-ownership.patch	\
   %D%/packages/patches/network-manager-plugin-path.patch	\
   %D%/packages/patches/newlib-getentropy.patch			\

gnu/packages/patches/neovim-tree-sitter-grammar-path.patch

@@ -0,0 +1,33 @@
+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)

gnu/packages/vim.scm

@@ -801,7 +801,8 @@ is based on Vim's builtin plugin support.")
               (file-name (git-file-name name version))
               (sha256
                (base32
-                "1b524vi44gkcsyy8w4jggvprwdsgy0gjprgxpyhh0dmqm47c0c48"))))
+                "1b524vi44gkcsyy8w4jggvprwdsgy0gjprgxpyhh0dmqm47c0c48"))
+              (patches (search-patches "neovim-tree-sitter-grammar-path.patch"))))
     (build-system cmake-build-system)
     (arguments
      (list #:tests? #f
@@ -852,6 +853,19 @@ is based on Vim's builtin plugin support.")
                   lua5.1-libmpack
                   tree-sitter))
     (native-inputs (list pkg-config gettext-minimal gperf))
+    (propagated-inputs
+     ;; bundled tree-sitters, neovim assumes that these are available.
+     ;; See https://neovim.io/doc/user/treesitter.html#treesitter-parsers
+     (list tree-sitter-c
+           tree-sitter-lua
+           tree-sitter-markdown
+           tree-sitter-vim
+           tree-sitter-vimdoc
+           tree-sitter-query))
+    (native-search-paths
+     (list (search-path-specification
+            (variable "TREE_SITTER_GRAMMAR_PATH")
+            (files '("lib/tree-sitter")))))
     (home-page "https://neovim.io")
     (synopsis "Fork of vim focused on extensibility and agility")
     (description