[Feature] Alias for combinations (#224)

[lheiskan]

Extends [key-mapping.key-notation-to-key-code] to support defining aliases for modifier combinations (e.g., hyper key).

Example config:

[key-mapping.key-notation-to-key-code]
hyper = 'ctrl-alt-shift-cmd'

[mode.main.binding]
hyper-c = 'workspace c'

Mixing modifiers and keys in a single alias is not supported. Invalid combinations are checked and result in configuration error messages.

Change introduces KeyOrModifier enum to represent either single keys or modifier combinations.

Implements: #224
Related: #389
Related: #1223

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[nikitabobko]

I didn't yet have time to review this PR. But just wanted to mention that this PR has heavy merge conflicts with another PR: #1714

The mentioned PR is more important and will be reviewed/merged first. If the conflicts are trivial, I will resolve them myself. Otherwise, this PR might require more adjustments in the future

[lheiskan]

Had a look at adding these changes to PR: #1714 - i can update this pull request once #1714 is merged, if that helps. Cheers!