Support custom shortcut key commands

[youknow04]

This PR introduces basic support for custom shortcut keys in Aider.

Many programmers may love custom shortcuts, especially in terminal-based environments like Aider.

This feature allows users to define their own shortcut keys in the .aider.conf.yml configuration file. (1,2,3,4 are reserved)

For example:

short_key_1: "/run ./useful_script_like_rust_cargo_test_in_sub_dir_and_come_back.sh"
short_key_2: "refactor this code"
short_key_3: "/model gpt-4o"
short_key_4: "/model claude-3-opus-20240229"

then you can just type /1 or /2 in Aider.

The short_key_1 example is particularly useful for Rust programmers. Since Aider needs to be run from the root directory of the git repo, while running cargo test directly from the root will not work if the Rust code is located in a subdirectory.

The default behavior of Aider remains the same if no custom shortcut keys are configured.

[paul-gauthier]

Why not just use up-arrow and control-r to search for your previous messages/commands and re-send them?

[youknow04]

Shortkeys require less typing and eliminate the need to search through history external to our brain.

This is a basic supoort. Later, we may improve this by adding argv like /1 filename

[paul-gauthier]

I understand the need here, but I'm not sure I can adopt the /1, /2, /3 type syntax.