impr: Auto `@interpolate(flat)` when integer field encountered in varyings struct

[cieplypolar]

For example, when we define vertex output struct like the following

const ModelVertexOutput = {
  worldPosition: d.vec3f,
  worldNormal: d.vec3f,
  applySeaFog: d.u32, 
};

We will get the error, that integer fields must be marked with explicit @interpolate(flat)

I think, it won't harmful to do it automatically for integer types.