context powerPreference should default to high-performance

Web browsers moved to change the default WebGL powerPreference from high-performance to low-power (e.g. in Chrome 80). This caused a regression in performance for Unity WebGL content on laptops with two GPUs (a low-power integrated GPU and a dedicated GPU) where now the low-power GPU is selected.

We need to default to selecting the high-power powerPreference, but add an option to the WebGL Player Settings to allow the user to choose a different setting (default or low-power) if their content is more suited.

See:
https://forum.unity.com/threads/force-the-use-of-dedicated-gpu.1115851/
https://github.com/emscripten-core/emscripten/issues/10000
https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.2.1

Users can currently work around this by setting the config['webglContextAttributes'] when calling createUnityInstance from their WebGL template, and we should still respect that setting if set.