Add predicate for detecting holding up shields

I've been trying to make custom enchantment datapack in 1.21,and noticed that there is no way to detect players holding up shields. Detecting them is essential to make new shield enchantments. I suggest adding new player-specific predicate that detects holding up shields.

I know there is already similar feedback which suggests new scoreboard criteria, but using predicate might be simpler and easier way for making custom enchantments so I've created new feedback.