Add a way to detect the target and generate native code

[SirLynix]

The idea is to implement #63 but in another way:

fn main()
{
    const if (target(spirv))
    {
        // target is spirv
    }

    const if (target(glsl, 430))
    {
        // target is GLSL 4.3
        const if (target_ext("GL_EXT_shader_samples_identical"))
        {
            // GL_EXT_shader_samples_identical is supported
        }
    }
}

now that feature is mostly useless without a way to inject GLSL/SPIR-V/WGSL/... directly, a bit like asm blocks in C. However I'm not sure about the syntax.

const if (target(glsl) && target_ext("GL_MESA_shader_integer_functions"))
{
    // ?
}

I think the best way would be to have a special kind of function, maybe something like

[cond(target(glsl) && target_ext("GL_MESA_shader_integer_functions"))]
[asm(glsl)]
fn bitcount(value: i32) -> i32
{
    // GLSL code, value being available to it
    /// return bitCount(value);
}