Minor internal api cleanups to improve comments and avoid errors

[0xtygra]

Motivation

For a personal project I needed to generate the calldata for (but not actually fulfill) a listed NFT. The SDK has an internal api.ts that supports this but from my brief searching it looks like it isn't exposed.

Solution

I ended up forking the repo and using it locally. In doing so, i found that the JSDoc for api.getOrder and api.getOrders had an options.limit that was in fact omitted in the typescript. Additionally, passing an orderBy: "eth_price" to api.getOrder throws an error, so I also removed that as a configurable option.

A couple of other things i noticed:

• a .env.example would be great, as well as updating the contributing.md to call out the need for a .env to run the tests. In addition, unless it's already done so, exposing the calldata generation functions from api.ts on the exported OpenSeaSDK object would be great!

[0xtygra]

ran into this error when trying to pass "eth_price"

[ryanio]

here is what the docs say, maybe we should just update the typedoc to include this?

The order in which to sort the results. Default: created_date NOTE: If eth_price is used, asset_contract_address and token_id are required.
https://docs.opensea.io/reference/get_listings