deps: v8 - cosmetic build adjustments

this is in order to give a better development experience in MSVS
eventually should land in v8 trunk
tools: build adjustments for v8 3.29

- Getting it to compile and be sane
- Removed references to v8::Debug*
build: added `verbose` argument to build tools
build: fix or suppress warnings

- activated "treat warnings as errors"
src: re-implement debug-agent

- based on discarded v8 code