Add alpha mode implementation to shader_material_2d

[Leinnan]

Objective

Bevy 0.15 introduced new method in Material2d trait- alpha_mode. Before that when new material was created it had alpha blending, now it does not.

Solution

While I am okay with it, it could be useful to add the new trait method implementation to one of the samples so users are more aware of it.

[IceSentry]

This is already covered in more details in the mesh2d_alpha_mode example.

If we add an alpha mode to this example it should be AlphaMode2d::Mask(0.5), not Blend because it doesn't need blending. It should also point to the above example.

[Leinnan]

@IceSentry I would argue that is not covered for the Custom materials part. In 0.14 if you implemented a custom material that was by default a blend one, since 0.15 it changed. Other than that, I think it could be AlphaMode2d::Mask(0.5), I will change it later.