ts: fix Wallet class declaration

[callensm]

Previously, the Wallet type declaration at the package root was declared as a const of type NodeWallet. This doesn't work and causes typing issues since now Wallet is a variable and not an instantiable class.

I changed the declaration so that Wallet is now a "new class" that extends the functionality of NodeWallet allowing it to maintain its class identify and instantiability.

[devanshsanghvi]

Hello, is that pushed to the latest npm package ? I am trying to use it but getting errors -

`import {Wallet} from "@project-serum/anchor";

const wallet = new Wallet(USER_KEYPAIR);
`

throws this error: Type 'NodeWallet' has no construct signatures.

[callensm]

@devanshsanghvi this hasn't been merged into master or cut into a version package version yet so the changes are not available.