Consider configurable marking indicator as highlight group instead of icon

[miversen33]

As we finish up neo-tree integration (again), we need to start respecting the provided configuration from neo-tree. We can also begin setting up some configurations of our own. One of these might be the ability for users to specify how they want marked nodes to appear. Currently there is no choice here, we just use a diamond. But maybe we allow them to mark with a highlight group instead so they can have clear color indicators? 🤷