Add clusterfuzzlite workflow

[DavidKorczynski]

This adds fuzzing by way of ClusterFuzzLite, which is a GitHub action that will perform a short amount of fuzzing for new PRs. The goal is to use fuzzing to catch bugs that may be introduced by new PRs.

If you'd like to test this the way ClusterFuzzLite runs it (by way of OSS-Fuzz) you can use the steps:

git clone https://github.com/google/oss-fuzz
git clone https://github.com/DavidKorczynski/tinyexpr
cd tinyexpr
git checkout cflite

# Build the fuzzers in .clusterfuzzlite
python3 ../oss-fuzz/infra/helper.py build_fuzzers --external $PWD

# Run the fuzzer for 10 seconds
python3 ../oss-fuzz/infra/helper.py run_fuzzer --external $PWD fuzzer -- -max_total_time=10