Add support for webp extension texture in gltf importer.

[greatpxk]

Reason:
I encounted one problem that glb models generated by microsoft trellis 2.0 cannot be loaded. It shows an error:
Something went wrong
Failed to import model.
Unsupported extension: EXT_texture_webp.
So I decided to fix it.

Summary of Changes:
Added EXT_texture_webp to the list of supported extensions
Added 'EXT_texture_webp' to the supportedExtensions array
Handled the WebP extension in the texture import method
Checked whether the texture uses the EXT_texture_webp extension
If the extension is used, retrieved the correct image index from the extension

How it Works:
The EXT_texture_webp extension allows glTF files to use WebP format textures as an alternative to traditional formats like PNG or JPEG. When a texture object includes this extension, it specifies an alternative image source index that points to a WebP-formatted image.
WebP format is natively supported by browsers, so no additional decoding library is needed.

[greatpxk]

@kovacsv