Add customization capabilities to which-key menu

[xipeng-jin]

Closes #10910

Support parsing extra fields "binding_labels" and "group_labels" under each context stack in keymap.json

Release Notes:

• Added customization capability to which-key menu to enabling renaming action name and prefix of the group of actions.

[ConradIrwin]

@xipeng-jin Thanks for the idea!

I don't think we should have the abstraction leak such that the whole code-base needs to know about the which-key feature; instead if we want to do customization we should put it in that crate.

That said, I don't think we need to do this at all. Zed already has the concept of "namespaces" for actions (vim::Blah, editor::Blah), and introducing another grouping that is orthogonal to that seems a bit confusing. (even :help g in Vim gives you "commands starting with 'g'").