feat: Show filepath context in bufferline

[icorbrey]

This change adds functionality that computes the shortest unique filepath for each document in the editor to display as the title in the bufferline, along with the appropriate configuration options.

Configuration changes

editor.bufferline has been moved to editor.bufferline.show, and a new key editor.bufferline.context has been added with the following variants:

• "none": Don't provide any additional context in the bufferline. This maintains previous functionality.
• "minimal": Show the shortest unique filepath for each document in the bufferline. This is the new default.

Setting the render mode directly in editor.bufferline is still supported, so existing configs won't break.

Functionality

With editor.bufferline.context = "none", the following set of documents is mapped as follows:

Path	Bufferline text
book/src/editor.md	editor.md
helix-term/src/ui/editor.rs	editor.rs
helix-view/src/editor.rs	editor.rs

With editor.bufferline.context = "minimal", the same set of documents is now mapped as follows:

Path	Bufferline text
book/src/editor.md	editor.md
helix-term/src/ui/editor.rs	ui/editor.rs
helix-view/src/editor.rs	src/editor.rs

Verification steps

1. Set editor.bufferline.show to always.
2. Open multiple buffers pointed at the files with the same filename.
   • Those buffers should show enough of their files' paths to be unique.
3. Open another buffer with a different name.
   • That buffer should only show the filename.
4. Set editor.bufferline.context to none.
   • The buffer titles should now show the filename only.

Possible additions

We could add another context variant "full" which would map documents to their full path (either from Helix's working directory or, if outside that, from the user's home directory (e.g. ~/...) or from the system root (e.g. /...).