native SCIP support

[williamfzc]

#71

Looks like SCIP does ineed have this information under enclosing_range.

It can support func/class (semantic level) natively without sibyl2.

https://github.com/sourcegraph/scip/blob/main/bindings/go/scip/convert.go

• making sure the scip indexer will emit enclosing_range correctly
• add new fields in SourceContext for storing new values from SCIP
• API