Add a document describing our environment variable usage.

[grynspan]

Adds a document describing our environment variable usage for reference by developers working on Swift Testing.

This document is not an API contract: the set of environment variables Swift Testing uses may change at any time.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

I just landed #1287 and it added a new env var SWT_EXPERIMENTAL_EVENT_STREAM_FIELDS_ENABLED. Could we add that here before landing?

I imagine we could place it in a new section (with its own table) titled "Event stream" perhaps, and hoist SWT_PRETTY_PRINT_JSON there as well instead of having that be in the "Miscellaneous" section.

[grynspan]

@swift-ci test