Debugger should warn if function breakpoints are set

[gregg-miskelly]

This is copied from #295 -- when setting a function breakpoint, there is no good indication that these aren't supported.

[wesrupert]

There's no actionable event on our side. Looks like VS Code respects our supportsFunctionBreakpoints: false only on the protocol-level, not in the UI. Created Issue #9099 on their side, closing here.