Refactor particle_process_material to use c++11 long string literals

[Lcbx]

• Production edit: Follow-up to Use raw string literals for BaseMaterial3D shader code generation #89267.

simple refactor of ParticleMaterial shader to use use c++ 11 long strings

In pratice, for blocks of > 3 lines, use :

code += R"(
    my;
    long;
    piece of code;
);

instead of

code += "    my;\n";
code += "    long;\n";
code += "    piece of code;\n";

tested by creating different ParticleMaterials, converting them to ShaderMaterial and diffing the generated code with stock Godot
( it adds some endlines, I don't think that matters overmuch)

[Lcbx]

took me a bit to realize that coding style check was tripping over inconsistent indentation.