Improve GLSL classification

[smola]

Description

• added extensions .gs, .comp and .glslf.
  See: https://stackoverflow.com/q/6432838

• add disambiguation rule for .gs files:
  if it starts with #version <number>, it's GLSL.
  This seems to work for most GLSL files with few exceptions
  that are catched by the classifier.

• added sample extrude_normal.gs from kyle-piddington/ShaderTool
  (licensed under MIT):
  https://github.com/kyle-piddington/ShaderTool/blob/c753a53bde6eab942da617adab9483c945f27f51/assets/shaders/extrude_normals.gs

• added sample blend_120.glslf from gfx-rs/gfx (licensed under
  Apache 2.0):
  https://github.com/gfx-rs/gfx/blob/7b084cfbb95e3e4d1ea51e91746fc75b2efb3be3/examples/blend/shader/blend_120.glslf

• test_heuristics.rb now supports alt_name to ake a different file name
  for a fixture.

Checklist:

• Continuation of improve GLSL classification #3747 improve GLSL classification #4089 improve GLSL classification #4402.
• Last pending review at improve GLSL classification #4402. Now solved.
• After merging this, an issue should be opened to find proper disambiguation for .comp. See improve GLSL classification #4402 (comment)

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