FUTURE: stabilize some `Deno` APIs behind the flag

[Bartek Iwańczuk]

We would like to stabilize some of the Deno APIs behind DENO_FUTURE=1 env var, namely:

• All the file system APIs available in ext/fs
• WebGPU APIs in ext/webgpu
• Deno.dlopen and related APIs in ext/ffi

In other words, we don't want to perform check_unstable() calls in these APIs if DENO_FUTURE=1 env var is present.

This requires two separate changes:

1. For FS and FFI APIs we need to properly set up the unstable checker in cli/factory.rs to enable these two APIs if DENO_FUTURE is present
2. For the WebGPU API you want to enable it in exposeUnstableFeaturesForWindowOrWorkerGlobalScope in 99_main.js if the future setting in runtimeOptions is set to true.

We should add a single integration test that checks all these APIs being present without --unstable-* flags but with DENO_FUTURE=1 env var.

Skip changing type declarations for now.

[Ryan Dahl]

webgpu and ffi not done yet