Feature Request: Rework custom UI renderer

[gorillamoe]

Rename custom_format_buffer_line to buffer_name. The implementation should stay the same.

Add ui.render.icon which defaults to nil to have the default icon and highlights implementation (if not disabled by the user).

It should also accept a function that receives the same argument as buffer_name. If a function is supplied, the function should return an icon that should be displayed.

Add ui.render.diagnostics which defaults to nil to have the default diagnostics and highlights implementation (if not disabled by the user).

The argument already contains the diagnostics information, so the user can use that to implement a custom version to be displayed.