Stop-gap support for command sequences/macros in keymaps

[cessen]

In #500 and #165 people have been discussing wanting alternative behaviors to some of Helix's current commands. In all cases, the desired behaviors can be achieved by composing existing commands. In some cases we may want to add built-in commands with those behaviors, but in many cases we probably don't. Nevertheless, I think it's reasonable for some users to want those behaviors.

To support customizing behaviors that way, I'd like to suggest adding support for binding command sequences to keys, not just individual commands. Perhaps with the count of each command optionally specified.

Along with designing the core commands to be composable, this would allow users a lot of flexibility to easily and quickly customize Helix's behavior.

Alternatives

Instead of supporting this at the keymap level, we could delegate all responsibility for this kind of thing to the plugin system. This would make the barrier to customizing Helix's behavior--even in basic ways--much higher, but would keep the keymap system a little simpler.

[cessen]

After discussing on the matrix channel, we've all agreed that this shouldn't be the long-term solution. Eventually we want to support some kind of scripting language for configuration rather than TOML, and that would provide enough flexibility to do this (among other things) without needing to reach for compiled plugins.

However, we may still want to do this as a short-term solution, which will later get removed when we move to the new configuration system.