feat(config): support solc --experimental

[figtracer]

Summary

• Adds an experimental config option and --experimental compiler CLI flag.
• Maps Foundry config to solc Standard JSON settings.experimental through foundry-compilers.
• Updates config defaults, CLI parsing, and config output tests for the new field.

Dependency

Depends on the merged foundry-core compiler support from foundry-rs/foundry-core#94. This PR temporarily patches foundry-compilers to foundry-rs/foundry-core@39f19b016c2ba793b24255bda83bd3822c8cb525 until the next foundry-compilers crate release.

Context

Solidity 0.8.35 introduced experimental mode via --experimental and settings.experimental. Foundry should expose the flag while using typed compiler settings instead of injecting the flag into raw solc extra args.

Reference: https://www.soliditylang.org/blog/2026/04/29/solidity-0.8.35-release-announcement/

Validation

• cargo +1.95.0 test --locked -p foundry-config solc_settings_include_experimental_setting
• cargo +1.95.0 test -p foundry-cli can_parse_experimental
• cargo +1.95.0 test -p forge can_set_config_values
• cargo +1.95.0 test --locked -p forge can_inspect_standard_json
• cargo +1.95.0 fmt --check (passes; stable rustfmt emits warnings for nightly-only rustfmt options)