Add Jupyter DAP handler

[lionel-]

Branched from #1170
Backend side of posit-dev/positron#13205
Part of posit-dev/positron#12104

Implements notebook debugging via the Jupyter Debug Protocol. DAP messages flow through debug_request/debug_reply on the Control socket, with events forwarded as debug_event on IOPub.

Since notebook cells aren't files on disk, the Jupyter Debug Protocol uses dumpCell to write cell source code to deterministic temp files (path derived from a MurmurHash2 of the code). The frontend sends dumpCell before setBreakpoints, so breakpoints reference these temp files.

When execute requests contain a cellId, breakpoints and line directives are mapped to the temp files written by dumpCell. This allows users to execute and inject breakpoints at the same time, outside of a debugging session. Otherwise they'd need to start a debugging session with Debug Cell, then go back and reexecute cells containing breakpoints.

In addition:

• Interrupts in notebook mode now quit the debugger with Q if active. This fits the notebook UX because, unlike in Console mode, the kernel remains busy for the duration of the debugging session, and the notebook UI shows an Interrupt button next to the cell. If we don't quit the debugger, the cell will remain in busy state after interrupting, which is confusing.

• Debugging prompts created via browser() or debug() (as opposed to breakpoints) map to StdIn. Not the best UI in Positron (stdin prompts show via command palette) but much better than doing nothing. The alternative would be to initiate a full debug session from the backend, but this would require non-standard frontend changes.

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