build: update forge-std to v1.8.1

[smol-ninja]

Changelog

• Removes @prb/test dependency.
• Updates forge-std to v1.8.1.
• Since foundry native assertions are declared as pure, some of the test functions have also been declared as pure .
• Since the visibility of vm is private in StdAssertions, an instance of vm has been added in Base_Test.
  Ref: Change vm visibility to internal in StdAssertions and StdUtils foundry-rs/forge-std#530
• Defines MIN_INT256 and MAX_INT256 in Base_Test.
• Adds gas-custom-errors: "off" in .solhint.json.
  Ref: False version warning by gas-custom-errors rule protofire/solhint#570

[PaulRBerg]

Thanks for this PR and the clear changelog report, @smol-ninja. Also, kudos to your open-source citizenship.

Defines MIN_INT256 and MAX_INT256 in Base_Test

Why? Does Forge Std no longer provide them?