Check semgrep rules at initialization time and error out if incorrectly formatted

Release notes

Problem to solve

Semgrep rules are a separate concern from the semgrep engine, which is great. However, right now writing rules requires an engineer to execute a rule candidate against a test project, delaying feedback until the scan is complete.

Proposal

Introduce a new feature - either in the GitLab wrapper around semgrep or in semgrep itself - that checks the format of rules as the scanner is being initialized.