fix: allow "0%" snap points

[richkeenan]

Motivation

I setup my snap points using percentages, ["0%", "10%", "60%"] but got an error saying

Invariant Violation: '0%' is not a valid percentage snap point! expected percentage snap point must be only numbers and '%'. e.g. '50%'.

This is because the invariant uses the result of the Number constructor on the part before the percentage sign - which returns 0 here which is falsey.

This is easily fixed by using the number 0 rather than the string percentage but it took me a minute or two to realise I could mix numbers and % as my head was in 'percentage mode', so I thought a PR to allow "0%" might save someone else a few minutes of their time. This PR checks Number.isNaN on the output of the Number constructor which allows for 0 and any other legitimate number.

[gorhom]

thanks @richkeenan for submitting this pr 👏