Design Doc: v8::String::WriteUtf8() without flattening

[Caitlin Potter]

I've shared this on Slack already, hoping to get a few more eyes on it via v8-dev. https://docs.google.com/document/d/1jn6KFlrnfGwI1f7eLS7QQ7STzN58voONCk4TuCMt7es/edit?usp=sharing

The primary goal here is to enable the option to read or process string contents safely in embedder functions which take advantage of the FastApiCall mechanism, without modifying the v8 heap (which would be unsafe for such functions, but is not explicitly restricted, currently).

Please take a look at your leisure.

[Ben Noordhuis]

Embedder data point: String::Flatten() comes up frequently in Node.js
performance profiles so this would be a welcome change (assuming it's
faster, which, for many inputs, I expect it will be.)