Enable virtualtext by default

[w0rp]

Vim 9 and recent NeoVim versions have generally pretty good virtualtext support. We could start enabling virtualtext messages by default with clear documentation for how to turn the information off easily so the default experience for new users is that warnings and errors start appearing in their editors in a place that's very easy to view.

The demo GIF in the ALE README could be updated to show virtualtext errors and warnings, plus :ALEFix, and maybe some completion being done too.

[w0rp]

I might set it so the default is based on a has check for new enough Vim and NeoVim versions.

[yous]

With let g:ale_set_highlights = 0, this causes the error E970: Unknown highlight group name: 'ALEVirtualTextError'.

[marcinn]

May I suggest disabling this "feature" at default? It generates a mess in a editor window, which reduces code readability. The window with issues / statusline are good enough.