EIP-3860 Initcode

Documentation for tests/shanghai/eip3860_initcode.

Generate fixtures for these test cases with:

fill -v tests/shanghai/eip3860_initcode

abstract: Test EIP-3860: Limit and meter initcode

Tests for  [EIP-3860: Limit and meter initcode](https://eips.ethereum.org/EIPS/eip-3860).