Add $idx builder method to bindGroupLayouts

[mhawryluk]

closes #523

• doesn't do anything yet
• isn't generated by tgpu-gen (also yet)

[iwoplaza]

🥑🥑🥑

[iwoplaza]

Tiny nit, we might be able to rephrase this to also note on the automatic indexing if the method is not called.

Suggested change

	/**
	* Marks a bind group layout to appear with a specific group index in the resulting WGSL code
	*/
	/**
	* Associates this bind group layout with an explicit numeric index. When a call to this
	* method is omitted, a unique numeric index is assigned to it automatically.
	*
	* Used when generating WGSL code: `@group(${index}) @binding(...) ...;`
	*/