Expose commands and keybindings to plugins

Please expose commands and their key bindings to Lua plugins.

Commands include Micro built-ins (set, bind, run, help, tab...) and those installed by plugins (jumptag, references, menuedit...).

Keybindings include defaults (Ctrl-S save) and those defined by user and plugins (F1 command:cheat).

Plugins should be able to read the actual, current situation of all commands and all keybindings in Micro.

Use case

I wrote Palettero command palette plugin. As seen in bug #523 Suggestion: Command Search-As-You-Type, there is demand for command palette in Micro.

Palettero would need to get fresh list of commands and keybindings to show them to user.

Currently, I was able to retrieve runtime files, such as colorschemes and help files, but not commands and keybindings. For commands, I had to resort to just showing built-ins but not those defined by plugins.

Start by tracing Micro's Lua plugin API and the code that registers built-in and plugin commands, default and user keybindings. Determine how to expose the current command and keybinding sets, including plugin additions and user overrides. Done means a plugin such as Palettero can retrieve fresh lists of all commands and keybindings.