inspector: added inspector.waitForDebugger()

This method blocks current node process until a client sends Runtime.runifWaitingForDebugger.

It can be useful when we need to report inspector.url() before waiting for connection:

inspector.open(0, undefined, false);
fs.writeFileSync(someFileName, inspector.url());
inspector.waitForDebugger();

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines