Adding type signatures to `#audit_formula` methods in formula cops
would lead to verbose, repetitive signatures across the existing ~63
instances. This reworks `#audit_formula` to use a `T::Struct` for its
arguments, which allows us to use a one-line signature for these
methods.
Formula caveats text appears on formulae.brew.sh but escape
characters, as used in ANSI escape codes, should not appear in HTML.
This commit adds a RuboCop to disallow escape characters in the
caveats text.
