Dedicated keybind for shield activation.

There should be an option in the keybind settings in both Java and Bedrock editions of Minecraft reserved for shield activation.

This solves two major problems:

1. In Java Edition, this avoids the right click conflict between mainhand and offhand items while using a shield.

2. In Bedrock Edition, this prevents sinking underwater while using the shield. As for touch controls, a special touch control button can appear on the screen (near the sprint-jump-sneak keys) only when there is a shield in the main or off hand.