Replace `partial_alpha` with `alpha_mode` field

[bardt]

As discussed in https://github.com/FraserLee/bevy_sprite3d/issues/12 there might be a need for more flexibility in which AlphaMode is applied to the sprite material. Having 2 parameters related to alpha would be confusing, so I decided to replace partial_alpha with an optional alpha_mode with docs matching in spirit.

Migration guide:

• replace partial_alpha: false with either alpha_mode: None or alpha_mode: AlphaMode::Mask(0.5);
• replace partial_alpha: true with alpha_mode: Some(AlphaMode::Blend).

[FraserLee]

Just a few small nitpicks, this looks great!

[FraserLee]

Thanks for the PR!

[bardt]

@FraserLee thanks for the quick responses and this amazing crate!