Fix compile failure in WASM without wgpu backend (#14081)

# Objective

- When no wgpu backend is selected, there should be a clear explanation.
- Fix a regression in 0.14 when not using default features. I hit this
compile failure when trying to build bevy_framepace for 0.14.0-rc.4
```
error[E0432]: unresolved import `crate::core_3d::DEPTH_TEXTURE_SAMPLING_SUPPORTED`
  --> /Users/aevyrie/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_core_pipeline-0.14.0-rc.4/src/dof/mod.rs:59:19
   |
59 |         Camera3d, DEPTH_TEXTURE_SAMPLING_SUPPORTED,
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `DEPTH_TEXTURE_SAMPLING_SUPPORTED` in `core_3d`
   |
note: found an item that was configured out
  --> /Users/aevyrie/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_core_pipeline-0.14.0-rc.4/src/core_3d/mod.rs:53:11
   |
53 | pub const DEPTH_TEXTURE_SAMPLING_SUPPORTED: bool = false;
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: found an item that was configured out
  --> /Users/aevyrie/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_core_pipeline-0.14.0-rc.4/src/core_3d/mod.rs:63:11
   |
63 | pub const DEPTH_TEXTURE_SAMPLING_SUPPORTED: bool = true;
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

## Solution

- Ensure that `DEPTH_TEXTURE_SAMPLING_SUPPORTED` is either `true` or
`false`, it shouldn't be completely missing.

## Testing

- Building on WASM without default features, which now seemingly no
longer includes webgl, will panic on startup with a message saying that
no wgpu backend was selected. This is much more helpful than the compile
time failure:
```
No wgpu backend feature that is implemented for the target platform was enabled
```
- I can see an argument for making this a compile time failure, however
the current failure mode is very confusing for novice users, and
provides no clues for how to fix it. If we want this to fail at compile
time, we should do it in a way that fails with a helpful message,
similar to what this PR acheives.

crates/bevy_core_pipeline/src/core_3d/mod.rs

@@ -49,7 +49,7 @@ pub const CORE_3D_DEPTH_FORMAT: TextureFormat = TextureFormat::Depth32Float;
 /// `sampler2DShadow` and will cheerfully generate invalid GLSL that tries to
 /// perform non-percentage-closer-filtering with such a sampler. Therefore we
 /// disable depth of field and screen space reflections entirely on WebGL 2.
-#[cfg(all(feature = "webgl", target_arch = "wasm32", not(feature = "webgpu")))]
+#[cfg(not(any(feature = "webgpu", not(target_arch = "wasm32"))))]
 pub const DEPTH_TEXTURE_SAMPLING_SUPPORTED: bool = false;
 
 /// True if multisampled depth textures are supported on this platform.