Cutom key definitions / add rhyper

Hi!

I know this has been asked before, especially before some other keywords/key definitions were added, but it was never added to skhd:

It'd be real nice to be able to create our own aliases for keys/key combinations.

My particular use case right now would be to add a `rhyper` key, meaning a `hyper` key using the right set of modifiers, so that I could use binds such as `rhyper + cmd - j`.

Start by locating skhd's existing keyword and key-combination definitions, then trace how configuration bindings are parsed. The requested behavior is complete when users can define aliases for keys or combinations and bind a right-modifier hyper key as `rhyper + cmd - j`; no file or test is named in the issue.

Written by the indexing model from the issue text.