StepSequencer: Editable BPM

[spen]

Right now the BPM of the sequencer is fixed 140 but it would be useful to allow the user to change this.

Lets make a new component that returns a nice looking dropdown box that hooks up to the sequencer state and updates it as the user interacts with it.

I updated #12 with a few options for component libraries, and added some details about the benefits of using one of them. We could use one of those and apply our own styles to it as we develop some themes.

In material, people would usually use a slider for numeric inputs: docs
Ant design's is a simple number field: https://ant.design/components/input-number/

[spen]

Here's the file (from my PR) that exports the state we need: useSequencerState.tsx(just noticed it should be a .ts).
And here's it being used in another component to tap in to this state: StepSequencer/index.tsx.
We just need to do the same, but use the second listed export like so:

// instead of just
const [ sequencerState ] = useSequencerState();

// We do the following, assigning the second listed value to `setSequencerState`
const [ sequencerState, setSequencerState ] = useSequencerState();

Now we can do setSequencerState( someNewState ) to replace the old state with some new state.