Add `#[warning]` attribute to mcdoc

[jacobsjo]

A #[warning] attribute to mark values that aren't forbidden by the game, but should be avoided anyways. Examples include values for the dust particle, and pre 1.17 weights in template pools.

Would probably want to have a way to specify a warning message - as doc comment or using #[warning=""], maybe.

[misode]

How would this look in the mcdoc source? Would we add another union member with the invalid range that has this attribute?

[jacobsjo]

Thats what I had in mind, so turn

weight: (
    #[until="1.17"] int @ 1.. |
    #[since="1.17"] int @ 1..150 |
),

into

weight: (
    int @ 1..150 |
    #[until="1.17"] #[warning] int @ 151.. |
),