TSL Transpiler: Add support for discard.

[Mugen87]

Related issue: https://discourse.threejs.org/t/how-to-use-discard-in-tsl/78217/2

Description

The TSL Transpiler can now transpile the discard syntax in GLSL to TSL:

if ( finalColor.a <= 0.0 ) discard;

is converted to:

import { Discard, If } from 'three/tsl';

If( finalColor.a.lessThanEqual( 0.0 ), () => {

	Discard();

} );

That said, there is a more compact way in writing the above statement in TSL without the If:

 finalColor.a.lessThanEqual( 0.0 ).discard();

But I think the above should be okay for transpiled code.