Expose tools for building & normalizing hotkey & sequence strings

[iansan5653]

This PR expands the usefulness of the hotkey package by exposing lower-level utilities so that external tools can work with hotkey strings directly instead of only binding event handlers. This closes #93.

The changes include the following:

New types NormalizedHotkeyString and NormalizedSequenceString. These are simple branded string types that should help to encourage the safe use of the hotkey & sequence strings. Normalized hotkey strings can be parsed from plain strings with normalizeHotkey or they can be obtained from events with eventToHotkeyString. This way, a comparison of string === NormalizedHotkeyString will obviously be false and cause a lint error, ensuring that consumers don't forget to normalize before comparing. The same applies to `NormalizedSequenceString.

Combined the sequence.ts and normalize-sequence.ts files since they are closely related. This groups the code that creates sequence strings together.

Added a new normalizeSequence helper that is the sequence form of normalizeHotkey, enabling abstracting away the splitting and rejoining of sequences. Also added a SEQUENCE_DELIMETER constant.

Exposed all exports from sequence.ts and hotkey.ts publicly.

[theinterned]

Given this is a breaking change, perhaps we should target this change a v3-candidate branch to keep the main branch deployable?

[theinterned]

I added this PR to #74

[theinterned]

I really like the branded type use here. Very cool