Feat: forbid tasks on WebGL

[NickKhalow]

Context / Problem
DOTween exposes async/Task-based instructions when targeting .NET 4.6+ / .NET Standard 2.0.
On WebGL, Task-based async APIs are not supported in a meaningful or safe way due to the single-threaded execution model and lack of real threading. This leads to silent hangs or undefined behavior rather than explicit failures.

What was changed

• Excluded DOTween async instructions from compilation on UNITY_WEBGL.

• Added !UNITY_WEBGL to the existing preprocessor guards for async-related code paths.

• Applied consistently across:

  • UnityExamples
  • UnityTests (multiple Unity versions)
  • Prebuilt DOTween assembly sources

Why this approach

• Enforces correctness at compile time, not runtime.
• Prevents accidental usage of unsupported async APIs on WebGL.
• Aligns with WebGL platform constraints and avoids silent runtime failures.

Impact

• No behavior change for non-WebGL platforms.
• On WebGL, async/Task-based DOTween APIs are no longer available, making platform limitations explicit.

Risk
Low. Change is limited to conditional compilation and does not affect runtime logic on supported platforms.