Batch writes

[simolus3]

Another attempt for #3321, my previous PR for this got stale, this one is a bit simpler.

This attempts to solve the issue of build_runner and external tools watching for file system events (like the analysis server) not playing too well together. At the moment, a common issue is that, as a user runs build_runner build and the generated assets are written as they are generated, the analysis server will see many file changes and keep re-analyzing sources in an incomplete state.
By making build_runner cache pending writes in memory and then flushing them after a completed build, we'll likely get all changes to disk before they're picked up by external tools, reducing friction.

I've implemented this at a fairly low level (the raw file readers and writers making up the default IOEnvironment). They have to opt-in to batches by checking for the presence of a zone key when reading or writing assets. We support batches by default when not in low-resources mode. A batch runs in a zone wrapping a whole build run.

A todo is that I need to write integration tests for this, but it would be good to get a review for the overall approach before completing this.

Closes #3321.

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[jakemac53]

@simolus3 looks like some test utilites also need to implement the new method :)

[simolus3]

I'll need someone to approve the workflow again :)

[jakemac53]

Given this is effectively tested by the regular integration tests, I am fine with landing this now if you are. Ready for me to merge?

[simolus3]

I'm fine with the current state too 👍 I mainly had a "no files are visible before the build has completed" idea for integration tests, but since we have unit tests for the functionality and integration tests ensuring this didn't break anything, I think this is ready.