Add forge lint config

[ScreamingHawk]

This config suppresses warnings about the use of keccak256 and variable name casing.

Intentionally missing Info level severity. Otherwise we need this to cover all warnings thrown:

exclude_lints = [
  "asm-keccak256",
  "mixed-case-variable",
  "mixed-case-function",
  "screaming-snake-case-immutable",
  "screaming-snake-case-const",
  "unaliased-plain-import",
]