Show source for rule file locations

[robrix]

When we show an error indicating a given location in a rule file, I'd like to see an excerpt of the source around that location. One such should be shown for each rule file location involved in the error (tho maybe we should filter for duplicates).

I don't know if we should also show an excerpt for non-error locations, e.g. "Executing edge … -> … at (x, y)" entries. Perhaps if they're the last one before a source file (syntax node) location?