Introduce an outline panel

[SomeoneToIgnore]

Adds a new panel: OutlinePanel which looks very close to project panel:

has similar settings and keymap (actions work in the OutlinePanel context and are under outline_panel:: namespace), with two notable differences:

• no "edit" actions such as cut/copy/paste/delete/etc.
• directory auto folding is enabled by default

Empty view:

When editor gets active, the panel displays all related files in a tree (similar to what the project panel does) and all related excerpts' outlines under each file.
Same as in the project panel, directories can be expanded or collapsed, unfolded or folded; clicking file entries or outlines scrolls the buffer to the corresponding excerpt; changing editor's selection reveals the corresponding outline in the panel.

The panel is applicable to any singleton buffer:

or any multi buffer:

(search multi buffer)

(diagnostics multi buffer)

Release Notes:

• Added an outline panel to show a "map" of the active editor

[nervenes]

thanks for this! 🙏

is the font size intentionally larger than everything else? is it possible to match it with the buffer_font_size or ui_font_size?

[SomeoneToIgnore]

Thank you, nothing is final here even if intentional, as the first version is a draft to iterate on mainly.
I've tried to keep outline items rendered similar to the outline modal items, but apparently did indeed used a wrong size, will try to find and fix it soon.