feat: support web in package:platform

[vaind]

platform provides a mockable interface for dart:io platform. I've suggested to the team that we use it to replace our custom implemention of the same concept with this package.

However, I've failed to notice that pub.dev/packages/platform does not support web.

My suggestion is to add this, even if it's just with the information that can be gather with what is available on web.
Note: I have not actually researched and implemented any of the specific getters, just wanted to verify that it would make sense and wanted to show how it could look like in the code, with the conditional export.

If maintainers are open to introducing dart2js & dart2wasm support, I could update the code in local_platform_web.dart and add tests

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