Implement `blend_src` attribute.

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(d.vec4f);

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

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

const fragmentFn = tgpu.fragmentFn({
  in: {},
  out: FragmentOutput,
})(() => {
  // ...
  return {
    a: d.vec4f(),
    b: d.vec4f(),
  };
});

Start with the WGSL blend-src attribute and WebGPU dual-source blending specifications, then trace the proposed data.dualSourceBlend API and its use as the fragmentFn output type. Define the supported type and member-name behavior from the issue, enforce the extension, placement, two-entry, index, and shared-type rules, and verify the example output shape.