dillon1000/react

[Flight] Let environmentName vary over time by making it a function of string (#29867)

This lets the environment name vary within a request by the context a
component, log or error being executed in.

A potentially different API would be something like
`setEnvironmentName()` but we'd have to extend the `ReadableStream` or
something to do that like we do for `.allReady`. As a function though it
has some expansion possibilities, e.g. we could potentially also pass
some information to it for context about what is being asked for.

If it changes before completing a task, we also emit the change so that
we have the debug info for what the environment was before entering a
component and what it was after completing it.