neovim: Conceal markdown links without vim-markdown

This also conceals some other stuff like JSON strings, but seems to be
worth it overall.

home/neovim.nix

@@ -103,6 +103,11 @@ in
     };
   };
 
+  xdg.configFile."nvim/after/syntax/markdown.vim".text = /* vim */ ''
+    syn region markdownLink matchgroup=markdownLinkDelimiter start="(" end=")" contains=markdownUrl keepend contained conceal
+    syn region markdownLinkText matchgroup=markdownLinkTextDelimiter start="!\=\[\%(\%(\_[^][]\|\[\_[^][]*\]\)*]\%( \=[[(]\)\)\@=" end="\]\%( \=[[(]\)\@=" nextgroup=markdownLink,markdownId skipwhite contains=@markdownInline,markdownLineStart concealends
+  '';
+
   programs.neovim = {
     enable = true;
     extraConfig = /* vim */ ''
@@ -124,6 +129,7 @@ in
       set tabstop=4                  " Show a tab character as 4 spaces
       set softtabstop=0              " Edit soft tabs as if they're regular spaces
       set shiftwidth=4               " Make autoindent appear as 4 spaces
+      set conceallevel=2
 
       set foldmethod=indent
       set foldlevelstart=99