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

[indecisivedragon]

a few issues (ex: #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.