Use DecompressionStream for decompressing .spz files

[mrxz]

This PR replaces the usage of fflate with DecompressionStream for decompressing .spz files. This lets the browser handle the decompression, yielding a noticeable improvement in loading times (measured in Chromium):

Example	Main	DecompressionStream
Hello World	341 ms	177 ms	-48%
WebXR	404 ms	206 ms	-49%
Splat Transition	284 ms	127 ms	-55%
	378 ms	195 ms	-48%
	837 ms	470 ms	-44%

⚠️ The Compression Streams API is inherently asynchronous, but Gunzip from fflate allowed for synchronous usage. This means that the interface for the SpzReader had to change. Since it's part of the exported symbols, this impacts all user-code that use SpzReader directly.

Before

const spz = new SpzReader({ fileBytes });
const numSplats = spz.numSplats;
// ... (allocate resources)
spz.parseSplats( ... )

After

const spz = new SpzReader({ fileBytes });
await spz.parseHeader(); // Required to access properties like `numSplats`
const numSplats = spz.numSplats;
// ... (allocate resources)
await spz.parseSplats( ... )

[dmarcos]

thanks