bug#58431: 29.0.50; [Eglot] Add "breadcrumb.el" feature based on imenu.el/project.el services

[Yuan Fu]

> In the original Eglot request, users are requesting this UI feature for
> the Eglot LSP client.  But this doesn't have any LSP-specific
> interactions.  It can and should be done entirely by using the services
> provided by imenu.el and project.el.
> 
> Eglot already integrates with Imenu, so as soon as this hypothetical
> breadcrumb.el is in place, then Eglot users can take advantage of it.
> 
> This is similar to which-func.el, which displays the trailing element of
> the breadcrumb in the headerline.  Perhaps the breadcrumb feature can be
> thought of as an augmentation of which-func.el's functionality.  I don't
> have a strong opinion there.

I think which-func is closer to breadcrumb conceptually and api-wise: Both 
which-func and breadcrumb are about the current function, while imenu is about 
a list of _all_ functions/sections. We could just allow which-func-function to 
accept a list of function names. In fact, the which-func function of 
tree-sitter powered python.el already generates a breadcrumb, I just joined all 
the function names along the path with “/“. 

> 
> João
> 
> PS: Eli, I'm CCing you to underline this as just an example of a UI
> feature that is requested for Eglot, but belongs somewhere else.  In
> this case, I believe there is 0 additional integration needed in
> eglot.el: it can be realized independently of the backend which feeds
> into Imenu (perhaps tree-sitter?) and project.el.

I agree, Emacs provides the framework/UI, and tree-sitter/eglot/future magic 
plug in.

Yuan