add semgrep rule to ensure we are using fd_cache for file handling

a few issues (ex: https://github.com/crashappsec/chalk/issues/225) have come up due to improper file handling

In chalk, we should always be using the `fd_cache` api to handle files, since it ensures that opened files are actually closed later so that we don't stall on too many files open or try to read from a closed file. To ensure that `fd_cache` is used, we should set up semgrep with a custom rule to check for the usage of other nim file handling and complain if detected.