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
Some ServiceWorker operations and methods require normalized URLs.
Previously, the generic `string` type was used.
This commit introduces a new `NormalizedUrl` type, a special kind of
`string`, to make this requirement explicit and use the type system to
enforce it.
PR Close#37922