#[cfg(platform)] for Feature Flags

## Feature Request

Dioxus runs on many platforms and OS, and I'm sure with different versions of the same Operating Systems or in case of the web then multiple Browsers / Renderers, this creates alot of confusion when one has to chase `caniuse.com` for Web and other vendor specific docs to handle for specific OS

## Implement Suggestion

Dixous can take this a bit further for help with DX.

We can easily have multiple platforms: "web", "blitz", "native", "desktop", "mobile", "xr" and so forth (similar to `#[cfg(feature = "")]`)

Example API:

Web Browser / User Agent: `#[cfg(platform = "web", browser = "")]`
iOS: `#[cfg(platform = "mobile", os = "ios", min_version = "18.3")]`
iPadOS: `#[cfg(platform = "tablet", os = "ipados")]`
Desktop (i.e. Windows 8 to 11): `#[cfg(platform = "desktop", os = "windows", min_version = "8", max_version = "10")]`

Creating fine-grained cfg handling for platform specific features, we could also make the second and any consecutive attributes optional.