feature: make describe-bindings more feature-rich.

[sakurawald]

In lem, we have 2 commands to describe the keymap.

1. The describe-bindings command: This command list the information key-to-command for all enabled modes at current point.

2. The documentation-describe-bindings command: This command list the information command-to-key(s) and command doc-string for all well-known commands. (Also, you can mouse-click to goto the definition of the command)

As we can see, the documentation-describe-bindings is more feature-rich, but it only support well-known commands.
The describe-bindings command does support to list all key-to-command provided by all enabled modes at current point, but it doesn't support some good features like display the doc-string of the command, allow mouse-click to goto the definition of the command.