Prevent parallel wasm builds

[bugQ]

Attempting to target wasm with the parallel feature on fails because rayon does not natively support it.

Parallelism on WebAssembly is technically achievable through the Web Workers API, but until such functionality is stable, this should provide a better error message.

[Vrixyz]

I believe since rayon-rs/rayon#1110 ; rayon has a synchronous fallback to work on web ; needs verification though.

[Master-Hash]

needs verification though

I confirm rapier.js with parallel feature with wasm-bindgen-rayon works on web.

Both multi thread with Web Workers, and single thread fallback works.

According to

rapier/src/pipeline/physics_hooks.rs

Lines 138 to 157 in 6265129

	// TODO: right now, the wasm version don't have the Send+Sync bounds.
	// This is because these bounds are very difficult to fulfill if we want to
	// call JS closures. Also, parallelism cannot be enabled for wasm targets, so
	// not having Send+Sync isn't a problem.
	/// User-defined functions called by the physics engines during one timestep in order to customize its behavior.
	#[cfg(target_arch = "wasm32")]
	pub trait PhysicsHooks {
	/// Applies the contact pair filter.
	fn filter_contact_pair(&self, _context: &PairFilterContext) -> Option<SolverFlags> {
	Some(SolverFlags::COMPUTE_IMPULSES)
	}
	/// Applies the intersection pair filter.
	fn filter_intersection_pair(&self, _context: &PairFilterContext) -> bool {
	true
	}
	/// Modifies the set of contacts seen by the constraints solver.
	fn modify_solver_contacts(&self, _context: &mut ContactModificationContext) {}
	}

PhysicsHooks may not function, but I'm not its user anyway.

[bugQ]

Awesome, sounds like this PR is not needed then.