Detect Source effects

[lionel-]

Progress towards #1338
Branched from #1340

Moves source() into the effect abstraction introduced in #2, as the third effect kind after NSE and attach. A new SourceAnnotation handler replaces the hand-rolled path parser, and both the scan and the walk now recognize source() the same way they recognize library() or local(), through the shared resolve path instead of matching the name syntactically.

With these changes:

• source() can be shadowed by a binding or import
• The source annotation supports multiple files, to allow collated sourcing in the future, e.g. for targets::tar_source()
• The path argument is wired through CallContext::resolve_string(). It's currently inert but once data flow is hooked in we'll get: p <- "foo"; source(p)

Positron Release Notes

New Features

• Smarter handling of source() calls.

Bug Fixes

• N/A