This commit makes the `SwTestHarness.envIsSupported()` static method a
standalone function. This function is used to determine whether the
current environment provides the necessary APIs to run the SW tests and
is independent of `SwTestHarness`, so is no need for it to be a static
method of `SwTestHarness`.
This is in preparation of switching from our custom typings to the
official TypeScript typings (`lib.webworker.d.ts`).
PR Close#42736