Godot: new gdshader extension

[Rakka Rage]

#806

thanks for the godot support, looks great
shader already has a nice icon but godot is changing its shader extendion in 4.0 to gdshader
possible to make *.gdshader behave like *.shader please and thanks?

[John Gardner]

Added in 8707367; you'll see it in the next release (once I get around to cutting it).