Add option to show the path of the active tab in bufferline

[Sumandora]

This PR turns this: into this:

The main issue is that something like #8362 is not merged. I initially wanted to expand all tabs by default, however this means that you have a about 3 tabs before you go off-screen. I came up with this solution as a way to work-around this limitation.

To not change current behavior, the entire thing is hidden behind editor.expand-bufferline, which is false by default.

Paths that aren't part of the working directory are displayed as absolute paths, as opposed to relative paths.