Add a rule for std injection in edition 2018

I do not know how this was missing, I've looked at it a million times. I
feel like I was maybe missing something, but I really can't find
anything that directly refers to this, nor can I find any open issues.

I believe this is now done with this relatively subtle bit:
https://github.com/rust-lang/rust/blob/4d08223c054cf5a56d9761ca925fd46ffebe7115/compiler/rustc_builtin_macros/src/standard_library_imports.rs#L42

In the past it used to use a different approach of emulating `extern crate std as _;`:
https://github.com/rust-lang/rust/blob/c8cf9f5a025bb475804b5a90f54aca310b952526/src/libsyntax_ext/standard_library_imports.rs#L42-L46

Author: ehuss

src/names/preludes.md

@@ -109,6 +109,10 @@ By default, the [`std`] crate is injected into the [extern prelude], and all mac
 
 If the `no_std` attribute is specified, then the [`core`] crate is used instead of `std`, and similarly all macros exported from `core` are placed into the [`macro_use` prelude].
 
+r[names.preludes.extern.no_std.edition2018]
+> [!EDITION-2018]
+> Before the 2018 edition, `std` is also injected into the crate root. `core` is injected instead of `std` if `no_std` is specified. Starting with the 2018 edition, these are not injected into the crate root.
+
 r[names.preludes.lang]
 ## Language prelude