Add float (or finite) type

[davetapley]

Since we already have an integer type:

mobx-state-tree/packages/mobx-state-tree/src/types/primitives.ts

Lines 113 to 117 in 6a50d79

	export const integer: ISimpleType<number> = new CoreType<number, number, number>(
	"integer",
	TypeFlags.Integer,
	(v) => isInteger(v)
	)

Do we want one or both of these?

1. A float type, which would restrict against NaN.
2. A finite type, which would restrict against NaN and infinities (taking inspiration from isFinite).

I'll concede that both could be implemented with refinement, but so could integer and we already have that.

This would have saved me from a bug where I am using parseFloat and a NaN was sneaking in to my model.
It would be great if I could just specify float in my model and have MST throw, instead of having to check for NaN manually.

---

Happy to make a PR if it will be accepted 🤞🏻

[coolsoftwaretyler]

Hey @davetapley - sorry it took so long for us to get back to you.

We would happily accept a PR to this effect if you're still interested. If you've lost interest in this, I will keep the conversation open and hopefully get this on the roadmap overall + let you know when we release one.