Add hl group for indentmini plugin

[karamanliev]

Adds support for indentmini hightlight groups.

IndentLine and IndentLineCurrent highlight groups are not really used by indent-blankline plugin.

These were added with 3c19449, but they were left in indent-blankline's group after v4.

This PR moves them to a separate file, which will be automatically loaded for lazy.nvim users, if they have the plugin installed.

Cheers!