Adds ability to pause JavaScript debugger from d8 by defining a global function
`handleInspectorMessage` which should block waiting for a new inspector message,
and `send` it afterwards.

Additionally, adds a simple helper script that, when invoked via `websocketd`
as per instructions, can be used for debugging `d8` using Chrome DevTools
(inspecting script sources, pausing, stepping over, etc.).

Change-Id: Iee75fb4e3f2ccc8c8552c804fefaefb233d6b089
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1829221Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Ingvar Stepanyan <rreverser@google.com>
Cr-Commit-Position: refs/heads/master@{#64040}