Allow key remapping in config.toml to multiple modes at once

[Joao-Pedro-Castelli]

The idea is for there to be a section in the toml file for writing a single remapping that will applie to multiple modes at once. This first version only adds a "all_modes" section, but, in the future, there could be other combinations of modes for the remap to apply to. The name of the section could change also.
closes #14770

[ghuh]

As part of this commit, should probably mention this feature on the remapping page

[Joao-Pedro-Castelli]

Yes, I will add documentation.

[ghuh]

Awesome, thank you!

[RoloEdits]

I also wonder about a syntax like:

[keys."normal|select"]

This would give more granularity rather than "all modes", while still reducing duplication.

Have you explored alternate options?

[Joao-Pedro-Castelli]

Yes, we were discussing how to write sections that could apply to any combinations of modes. Using '|' seems good to me. Just need to think some details of implementation, like how to decide when on key remmaping overrides another on the same key when there could be sections like "normal|select" and "select|insert". Or if a key can only be configured one time per mode.
I'm open to any sugestions.