Implement blend_src attribute.

[iwoplaza]

Spec: https://www.w3.org/TR/WGSL/#blend-src-attr
https://www.w3.org/TR/webgpu/#dom-gpufeaturename-dual-source-blending

Rules:

• dual-source-blending extension has to be enabled,
• Can only appear as an attribute for a member of a fragment shader output struct.
• The struct containing members with this attribute has to have exactly two entries (one with @blend_str(0), and the other with @blend_str(1)).
• All the members must have same data type.

Are degrees of freedom are:

• The shared type both members will have.
• The names of both members.

Potential API:

import tgpu from 'typegpu';
import * as d from 'typegpu/data';

// Default names

/* TgpuDualSourceBlend<vec4f, 'src0', 'src1'> */
const FragmentOutput = d.dualSourceBlend(vec4f);

// Custom names, useful for WGSL -> JS generation

/* TgpuDualSourceBlend<vec4f, 'a', 'b'> */
const FragmentOutput = d.dualSourceBlend(vec4f, 'a', 'b');

const fragmentFn = tgpu
  .fragmentFn([], FragmentOutput)
  .implement(() => {
    // ...
    return {
      'a': vec4f(),
      'b': vec4f(),
    };
  });