DEBUGGER_.warn when skipping breakpoint because of RUBY_DEBUG_SKIP_PATH

Start by tracing how RUBY_DEBUG_SKIP_PATH affects breakpoint handling in the debugger. Add coverage for a breakpoint in a skipped path and verify that DEBUGGER_.warn is emitted only when the breakpoint would otherwise stop; run the relevant debugger tests to confirm the warning.

Your proposal
If a user sets a breakpoint in a path that is skipped because of RUBY_DEBUG_SKIP_PATH, it would be helpful to log a warning whenever the breakpoint would have stopped if the configuration wasn't there.

Additional context
Sometimes I set up RUBY_DEBUG_SKIP_PATH for the most common case, but eventually end up having to dig deeper than usual, and add a breakpoint in a skipped path. After the debugger failing to stop there for a couple of times, I realize the problem and change the configuration.

It would be useful for the debugger to tell me that I have a breakpoint in a skipped path, to help me realize my error faster.