ABIGEN v2 tracking issue

[fjl]

This issue is about finishing the work to ship ABIGEN v2. The prototype of it is in this pull request: #26782.

To get it ready we need:

• Rebase cmd/abigen, accounts/abi/bind: implement abigen version 2 #26782 branch on master
• Move the v2 stuff to its own package. Use accounts/abi/bind2 for now. Final package name TBD.
  • note this will require duplicating some stuff from accounts/abi/bind into the new package
• Adding tests. To test this, we should create an example contract binding and run all functions of bind2 against a simulated backend
  • transact
  • call
  • log filtering
  • log watching
• Code generation tests. These should just compare the generated code to a known output.
• Adding function GoDoc comments and examples
• Creating documentation page on geth.ethereum.org