Unify envmap textures and deprecate sphericalEnvMap

[mrxz]

Description:
Follow-up of #5453, also preparation work for #5449. In A-Frame environment maps were handled separately in both the standard and phong shader. Besides the code-duplication this also meant that there was no texture re-use between them or other textures.

This PR moves the envMap logic into utils/material and systems/material in a similar vein to other textures. The eventual texture loads now also go through the texture cache. The possible values for envMap has been broadened to support both cubemaps and equirectangular maps. The latter allows a fallback for the currently incorrect behaviour of sphericalEnvMap. There is one small breaking change, the envMap value now behaves closer to asset property types, meaning it now only supports ID based selectors and would otherwise assume a URL in case it's unwrapped (e.g. breaking envMap="#someElement > a-cubemap.active").

Changes proposed:

• Move loading of envMap to a common implementation in utils/material and systems/material making use of existing texture loading and caching.
• Expand the possible values for envMap to support both cubemap (six images/<a-cubemap>) as well as Equirectangular images.
• Deprecate sphericalEnvMap as Three.js dropped support for them long ago and A-Frame incorrectly uses them as Equirectangular images. Fallback is in place, logging a warning, but otherwise ensuring those expecting the equirectangular behaviour won't notice a regression.
• Added unit tests for utils/src-loader and shaders/phong

[dmarcos]

Thanks!