this allows plugins to add their own mappings (though they should really be using StringToItemParser) without needing any legacy numeric ID bullshit in the mix.
I've stuck to only doing this in the places where I'm sure we should never get false back. Other places I'm less sure of (and I found more bugs along the way).
This reverts commit 9b52af62b6f469771f2355a964b35ecffa5117d9.
We shouldn't assume that a string maps directly to a legacy ID,
because we might want string aliases (e.g. dark_oak_boat) which refer to
items that have a specific meta value in the MCPE system.
Overall, I want to get rid of the reliance on IDs here and register all
this stuff using closure callbacks on VanillaItems, so getting rid of
this assumption also serves that goal.
