Refactor shebang parsing to remove regex dependency (#5690)
## Summary
Similar to #5567, we can remove the use of regex, plus simplify the
representation (use `Option`), add snapshot tests, etc.
This is about 100x faster than using a regex for cases that match (2.5ns
vs. 250ns). It's obviously not a hot path, but I prefer the consistency
with other similar comment-parsing. I may DRY these up into some common
functionality later on.
