Put wgpu's default features behind a "wgpu_default" feature flag

[StT191]

This PR enables users of iced, iced_renderer and iced_wgpu to opt out of wgpu's default features. It introduces a new feature flag "wgpu_default" that enables the old behavior of enabling the wgpu dependency with it's default features. This is included in the iced default features to keep the existing behavior for users relying on the previous defaults.

This is however a minor breaking change for users who would previously set the "wgpu" flag manually as they would now have to set "wgpu_default" to get the same behavior.

Since wgpu version 25.0.0 the selection of supported backends that wgpu compiles with can be fully configured via feature flags. This PR enables users of iced and it's supporting crates to manually select what features they want to enable, not being forced in to the defaults.

This PR is in the same spirit as #2828 but takes a different approach in naming features while also adding the same features to iced_renderer and iced_wgpu.

[dtzxporter]

Dup of #2828

[StT191]

Dup of #2828

Yes I just had a look at your PR after seeing your comment. I took a slightly different approach. Therefore I'd still like to keep this PR open for now to add to the discussion.