Rework eglot's mode-line

Rework eglot's mode-line

Mimic flymake by replacing the old menus of the mode-line with
"context menus".  List all usefull commands under the main menu
(eglot-menu-map), and commands related to LSP debugging under the
project menu (eglot-debug-map).

* eglot.el (eglot-read-documentation, eglot-customize): New
commands.
(eglot-mode-line-string): New defcustom.
(eglot-menu-map, eglot-debug-map,): New variables.
(eglot--mode-line-props): Rework to use eglot-menu-map and
eglot-debug-map.
(eglot--mode-line-format): Use eglot-mode-line-string.

GitHub-reference: close https://github.com/joaotavora/eglot/issues/792