[glTF 2.1] Promoted Extension Functionality

# Promoted Extension Functionality

The functionality of the following ratified glTF 2.0 extensions has been promoted into the core standard in glTF 2.1:

- [`EXT_texture_webp`](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Vendor/EXT_texture_webp)
- [`KHR_materials_emissive_strength`](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_emissive_strength)
- [`KHR_mesh_quantization`](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_mesh_quantization)
- [`KHR_node_visibility`](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_node_visibility)

glTF implementations must support the functionality of these extensions in order to be compliant with glTF 2.1.

## Why this design

These extensions are considered to be broadly useful and widely supported. Their functionality either provides great benefit, is easy to implement, or both.

Exact syntax of how these features are used with glTF 2.1 assets will be defined later.

### `EXT_texture_webp`

- Guaranteed WebP support would allow asset authors to ship WebP textures without a fallback in glTF 2.1 assets without portability concerns.
- The `image/webp` Media Type is officially registered with IANA (see https://www.iana.org/assignments/media-types/image/webp) and the WebP bitstream specification has been published by IETF (see https://www.rfc-editor.org/rfc/rfc9649.html).
- WebP image format support is considered baseline across web browsers since September 2020.

### `KHR_materials_emissive_strength`

- This ensures that glTF 2.1 asset authors can use emissive factor values greater than 1.0.
- The upper limit on the emissive factor values is an oversight in the glTF 2.0 specification.

### `KHR_mesh_quantization`

- This ensures that glTF 2.1 asset authors can use more optimal quantized types for vertex storage.
- The lack of quantized accessor types support for built-in vertex attribute semantics is an oversight in the glTF 2.0 specification.

### `KHR_node_visibility`

- This ensures that glTF 2.1 asset authors can use the standardized node visibility toggle.
- The standardized node visibility toggle provides considerable value for the new glTF 2.1 features and improves DCC workflows.

Start by reviewing the four linked extension specifications under extensions/2.0 and the glTF 2.1 standard context described in this issue. Define the exact glTF 2.1 syntax for each promoted feature; the work is done when all four extensions have an agreed, documented representation in the core standard.

Written by the indexing model from the issue text.