Reapply 3.2 version of package:platform with changes (#958)

Reapplies 0def20c4bef2b242db15b38202a54735101f5791

Completely new API which is cross-platform safe to use.

The `Platform.current` object exposes one (or potentially more)
of a `NativePlatform` (based on `dart:io`) or `BrowserPlatform`
(based on JS `window.navigator` or anything else relevant).
Currently no `Platform.wasm` or similar, but if a useful way
to recognize it is introduced, one can be added.

Each has its own API, and the current platform determines
whether a value of these types is available or not.
On unsupported platforms, a value is just `null`.

Provides a `testing.dart` library which exposes
`TestNativePlatform` and `TestBrowserPlatform`,
which are customizable classes that can be used for testing.
It's possible to run code with such a fake value as
the *current* platform (`Platform.current` is temporarily
updated to a value which exposes the, fx, `TestNativePlatform`
as its `Platform.nativePlatform`).

Still retains the legacy and deprecated API that makes
native properties directly available on `Platform`.
Code should migrate to the new API by doing:
* `const LocalPlatform()` → `Platform.current.nativePlatform!`
* `new FakePlatform()` → `new TestNativePlatform()`

It's currently not possible to have more than one non-`null`
platform-value.
Platforms are detected by the presence of the `dart:io` and
`dart:js_interop` libraries, and no current platform has both.
When such a platform exists, the code will need to be adapted to to
produce *both* (possible using conditional imports or conditional parts
and augmentations).

Updates from the original:

Split into abstract interface and config-imported implementation.
This should allow, for example, code-generators to refer to the
library of the public types uniformly across platforms.

Make `Platform.is<OS>` operations work on legacy `FakePlatform`.
Requires importing some testing classes into the main library,
which is not surprising when the legacy `FakePlatform` class
existed for testing to begin with.