Previously, many references would eagerly calculate their row/column in
the source code even if they did not cause an `Issue` to occur. This
requires reading the source code and calculating this information from
byte offsets from the swc node's `Span`.

This introduces `LazyIssueSource`, which simply wraps the `Source` and
the start/end byte offsets from the swc span. This is only converted to
an `IssueSource` when the issue's source location is accessed.