Add Imaginando VS 2 export target

[douglas-carmichael]

Imaginando VS 2 export target

Adds an Imaginando VS 2 export target to the standard PBR (material) and
Unlit (material_unlit) material nodes, plus a section in Exporting
Materials.

Imaginando VS 2
(Visual Synthesizer) is an audio-visual instrument whose materials are single
GLSL fragment shaders with a JSON manifest and a small predeclared environment
(time, alpha, color, resolution, texCoord, fragColor). The target
generates one .vs2.frag text file:

• the VS 2 JSON manifest (uuid via $uid(0), display name via
  $(file_prefix)),
• generated definitions via $definitions float_uniform_to_const,vs2 — the
  small custom vs2 filter renames elapsed_time to VS 2's time and bakes
  vec2/vec3/mat2 uniforms, in the same string-replacement style as the
  built-in unity/unreal filters,
• a void main() that evaluates the material's albedo, emission and opacity
  inside $begin_generate (VS 2 layers are unlit, so lighting inputs such as
  roughness/normal are intentionally not exported) and applies VS 2's layer
  alpha.

Testing

Byte-identical target definitions have been exercised as external export
targets (user://export_targets/*.mme) on macOS with Material Maker 1.7, both
from the GUI and via --export-material --target "Imaginando VS 2", and the
resulting files import and render in VS 2. Embedded and external targets share
the same code path in gen_material.gd, so behavior is identical.

Known limitations (called out in the documentation)

• $uid() emits 32 hex digits without hyphens, while VS 2 uses the canonical
  hyphenated UUID form; inserting hyphens in 8-4-4-4-12 grouping yields a valid
  v4 uuid. If there is interest, a follow-up could add a template function that
  emits the hyphenated form directly, which would remove this caveat.
• Materials using buffer-based nodes (blur, warp, …) cannot be expressed as a
  single fragment shader; such exports reference textures VS 2 cannot provide.