[experimental] Export yul cfg to json

[r0qs]

Depends on #15359

Test with:

solc --ir-optimized --optimize --yul-cfg-json --pretty-json test/benchmarks/verifier.sol

Or in assembler mode:

solc --strict-assembly --ir-optimized --optimize --yul-cfg-json --pretty-json test/libyul/yulStackLayout/if.yul

TODO:

• command-line tests
• Fix Yul switch statements (still contains GHOST and TMP slots)
  • TMP variables were removed, the GHOST variables may not be a problem after all.
• convert to SSA form

[ekpyron]

The existing tests for * output selection not changing confirm that this is hidden unless undocumented output selections or cli options are used, so I'm approving despite the lack of testing for now - we can let the GreY project play with it on the next release and work towards refining, stabilizing and testing it more properly later.