Convert @parity/api to TypeScript

[amaury1093]

It's the last package still in plain JS.

[amaury1093]

@yjkimjunior nice I just saw you assigned yourself, are you working on this?

Note: the way it's done right now, we can convert api to TS incrementally. See for example https://github.com/paritytech/js-libs/tree/42743c94a8c03c07f89ba0157874cc891e218cb3/packages/api/src, only util/ is in TS now. So feel free to tackle this bit by bit.

[pmespresso]

Had this on the backburner but working on it now. I'm starting by renaming files to .ts then seeing what breaks and then adding type definitions from there. Starting with building a interface for Contracts

[JamesLefrere]

@yjkimjunior Would you mind if I took this? Just my kind of thing :)

After this, it might also be interesting to explore adding a TypeChain target https://github.com/ethereum-ts/TypeChain

[pmespresso]

hey @JamesLefrere please go ahead, forgot to unassign myself

[ksato9700]

@JamesLefrere
Do you mind if I take the conversion of some components?