Add `Tag.update()` built-in function

[Nico314159]

Function signature:

Tag.update(selector: Selector, tag: Keyword)

Inspired by Trident

[Nico314159]

For example,

Tag.update(@e[nbt={OnGround:1b}], onGround);

compiles to

tag @e remove onGround
tag @e[nbt={OnGround:1b}] add onGround

[WingedSeal]

tag @e remove onGround looks really unoptimized?

[Nico314159]

tag @e remove onGround looks really unoptimized?

Well, the purpose is to remove it from all entities before re-adding to specific ones, so I don't know of a better way to do that.