refactor(shortcuts): Improve shortcut registry documentation & style

[cpcallen]

The basics

• [X] I validated my changes

The details

Proposed Changes

Make some minor refactorings of shortcut_registry.ts to improve readability and style, and add documentation for the KeyboardShortcut interface type.

Reason for Changes

It was a bit harder than necessary to grok how this worked (ideally one should not need to read the source code!), so trying to make it easier for others / future self.

Test Coverage

Passes npm test + quick manual verification that shortcuts still work.

Documentation

Improved, hopefully.

[cpcallen]

@rachel-fenichel: You are a spelling-error-detecting machine—and evidently I need to figure out how to get emacs to give me wavy red underlines…