Docs: Removed internal is* properties.

Describe limitation of point size

LightProbeHelper: Added doc page and TS file.

Add `internalFormat` to Texture

Docs: Added helpers in examples.

[GPUComputationRenderer] check if webgl2 is used before checking for extension OES_texture_float

Clean up.

Moved FaceNormalHelper, LightProbeHelper, PositionalAudioHelper, RectAreaLightHelper, VertexNormalsHelper and VertexTangentsHelper out of src

Moved PMREMGenerator to src/extras

TS: Clean up.

Some WebGL2 internal formats rely on new WebGL2 formats. This change
add the 4 new PixelFormat constants that are needed

* Adds `internalFormat` attribute to Texture.js and Texture.d.ts
* Adds `internalFormat` string mapping to GL constant in WebGLTextures.js
* Adds `internalFormat` in Texture documentation
* Adds internal format string constants to constants documentation