vim-plugins: honor the plugin manager re filetype and syntax

Currently, all the filetype and syntax are enabled *after* all the plugins has
already been loaded. Whilst this is the case for Pathogen, it's not
recommended when using VAM.

This commit applies the recommendation for:
- VAM[0]: The filetype and syntax are enabled *before* the plugins are loaded.
- Pathogen[1]: The filetype and syntax are enabled *after* the plugins are loaded.
- Plug[2]: The filetype and syntax are automatically enabled.

[0]: https://github.com/MarcWeber/vim-addon-manager/tree/d9e865f3c2de5d9b7eabbc976f606cf1b89e29ea#recommended-setup
[1]: https://github.com/tpope/vim-pathogen/blob/a553410f1bdb9000fbc764069f3a5ec3454a02bc/README.markdown#runtime-path-manipulation
[2]: https://github.com/junegunn/vim-plug/blob/2f5f74e5e67f657e9fdac54891a76721bcd3ead3/README.md#usage

pkgs/misc/vim-plugins/vim-utils.nix

···
       ''
         let &rtp.=(empty(&rtp)?"":',')."${vimPlugins.pathogen.rtp}"
         execute pathogen#infect('${pluginsEnv}/{}')
+
+        filetype indent plugin on | syn on
       '');
 
       /* vim-plug is an extremely popular vim plugin manager.
···
 
       in assert builtins.hasAttr "vim-addon-manager" knownPlugins;
       ''
+        filetype indent plugin on | syn on
+
         let g:nix_plugin_locations = {}
         ${lib.concatMapStrings (plugin: ''
           let g:nix_plugin_locations['${plugin.pname}'] = "${plugin.rtp}"
···
         set packpath-=~/.vim/after
         set packpath+=${packDir packages}
         set packpath+=~/.vim/after
+
+        filetype indent plugin on | syn on
       '');
 
   in writeText "vimrc" ''
-  " minimal setup, generated by NIX
+  " configuration generated by NIX
   set nocompatible
 
   ${vamImpl}
   ${pathogenImpl}
   ${plugImpl}
   ${nativeImpl}
-
-  filetype indent plugin on | syn on
 
   ${customRC}
   '';