webgl: Add feature guard - #47200

Add webgl feature guard, so that we can build without it.

Testing: has no automated tests, but can verify by building without the
webgl using `./mach build --no-default-features --features
bundled,gamepad,webgpu,max_log_level,js_jit`
Fixes: servo#47184

This PR does a lot of small changes:
- adds `webgl = []` as features to script, servoshell, servo,
constellation, paint
- adds a lot of `#[cfg(feature = "webgl")]`
- make webxr depend on webgl `webxr = ["webgl",` (as previouslty webgl
was non optional)
- adds `// skip-unless CARGO_FEATURE_WEBGL` to the `.webdl`'s

The .webdl part was tricky as it had:
```
typedef (OffscreenCanvasRenderingContext2D
  or ImageBitmapRenderingContext
  or WebGLRenderingContext
  or WebGL2RenderingContext
) OffscreenRenderingContext;
```
and as the WebGLRenderingContext is now optional, I had to introduce 

```
typedef (OffscreenCanvasRenderingContext2D
  or ImageBitmapRenderingContext
  // skip-unless CARGO_FEATURE_WEBGL begin
  or WebGLRenderingContext
  or WebGL2RenderingContext
  // skip-unless CARGO_FEATURE_WEBGL end
) OffscreenRenderingContext;
```

I though about making it a separate PR, but I guess it should stay here,
as the separate PR would need a working example anyway.

---

I made sure that this would still compile with and without webgl, and
currently webxr forces webgl as a dependency

---------

Signed-off-by: Asset Malik <5373400+janeoa@users.noreply.github.com>

This PR feature guards the devtools

Just like in the servo#47200 I introduced
a new feature `devtools`. It is now optional and requires `debugging`
(under the same feature guard as a direct dependency). This feature
guard preserves `console`.

PR also introduces a small `default_tooling = ["servo/default_tooling"]`
along to the already present `default_web_features`
```toml
...
default_tooling = ["devtools"]
...
```

(Cargo.toml of servoshell:)
```toml
default = ["default_without_allocator", "servo-allocator/use-jemalloc"]
default_without_allocator = ["bundled", "default_web_features", "default_tooling", "max_log_level", "js_jit"]
default_web_features = ["servo/default_web_features", "gamepad", "webgpu", "webxr"]
default_tooling = ["servo/default_tooling"]
```

Testing: `./mach build --no-default-features --features
bundled,gamepad,webgpu,max_log_level,js_jit`

Builds that pass:
- `./mach build --no-default-features --features
bundled,gamepad,webgpu,max_log_level,js_jit`
- `./mach build`
- `./mach build --features devtools`
- `./mach build --features webgl`

Fixes: [47187](servo#47187)

P.S. I've left a bunch of "dead_code" warnings unaddressed. Could be a
future PR candidate, but I guess the `production` profile would optimize
them away anyway and them extra feature guards might cause more clutter
in the default build?

---------

Signed-off-by: Asset Malik <5373400+janeoa@users.noreply.github.com>