Add `selectedSourceLine` and `selectedSourceColumn` for use in contextual queries

[aibaars]

Is your feature request related to a problem? Please describe.

For a "printCFG" query I'd like to be able to restrict the view to the Control flow graph of a single function instead of all functions in a file. Displaying all graphs of all files is too messy and hard to work with.

Describe the solution you'd like

In addition to the selectedSourceFile, I'd like the selected source line and columns to be available for use in contextual queries. That way a printCfg query can restrict itself to the enclosing function/callable at the position of the cursor.

Describe alternatives you've considered
none

see also: https://github.slack.com/archives/CPA9033L4/p1682926461306599?thread_ts=1682664735.271779&cid=CPA9033L4