strings arith checks preprocessing pass: step 1 (#5747)

strings arith checks preprocessing pass: step 1 (#5747)

We are adding a preprocessing pass that simplifies arithmetic constraints related to strings.
For example, len(s) >= 0 would be replaced by true.
This will make use of CVC4::theory::strings::ArithEntail::check.

This PR is the first step. It only includes the preprocessing pass infrastructure, with an empty implementation of the main function StrLenSimplify::simplify. It also adds the pass to the registry.
The implementation of this function is not complicated, but is left for a future PR in order to keep the PR short.

Future PRs will include an implementation of the main function, tests, and a command line option to turn on the pass.