[ethers] Mark exported ABI as const for better type analysis

[kdembler]

Hey, thanks for this awesome library!

After #368, contract ABI is exported as a static member on the contract's factory. However, because _abi that's defined in generated code is not marked with const assertion, ABI type definition is very simplified, as almost every field will be marked as string instead of string literal that narrows down possible values.

This is problematic because it won't allow some tools to take ABI from generated typechain bindings and extract rich data from it. For example, with release 0.7, wagmi library added support for type-safe contract calls, as long as abi passed is const asserted, allowing TS compiler to infer contract args, etc. You can find more info here: https://wagmi.sh/docs/typescript#type-inference

I propose as const is added to generated _abi code so that more type info can be extracted from bindings