Expose StripeResource on instance

[jlomas-stripe]

r? @stripe/api-libraries
cc @stripe/api-libraries @remi-stripe @paulasjes-stripe

This just surfaces the StripeResource on the instance as well, so you can continue to do const stripe = require('stripe')(apiKeyHere); but still be able to add resources while working with beta things.

[remi-stripe]

re-assigning to @richardm-stripe to both approve but also clarify if this should be done in the codegen part instead of here.

[jlomas-stripe]

I think it additionally needs a change to one of the preambles in the TS codegen stuff; not certain though.

[richardm-stripe]

I mean, users will get a type error if they try to use it and we don't add such a type. The problem is, the type of StripeResource is super complicated and difficult to type well -- this is an approximation, I think -- but it uses any a lot, and it only reflects the "public interface" of StripeResource...
ebc1a3a

We could just type it as any and call it a day, too. WDYT @jlomas-stripe

[jlomas-stripe]

Hey @richardm-stripe! I think in this case it's just a matter of adding StripeResource to here. since this PR exposes it there; I'm not sure if there's further typing that we can even do wrt custom StripeResource implementations.

[richardm-stripe]

Merging this without a typedef for now. (I don't think there's much advantage to adding StripeResource: any over omitting the typedef altogether)