src: simplify embedder entry point execution.

Previously we wrapped the embedder entry point callback into a binding and then invoke the binding from JS land which was a bit convoluted. Now we just call it directly from C++. The main scripts that needed to tail call the embedder callback now return the arguments in an array so that the C++ land can extract the arguments and pass them to the callback. We also set PauseOnNextJavascriptStatement() for --inspect-brk and mark the bootstrap complete milestone directly in C++ for these execution modes.