Rollup merge of #122060 - clubby789:stabilize-imported-main, r=lcnr

Stabilize `imported_main`

FCP: rust-lang/rust#28937 (comment)
Docs: rust-lang/reference#1461

Author: matthiaskrgr

test-cargo-miri/tests/main.rs

@@ -1,3 +1 @@
-#![feature(imported_main)]
-
 use cargo_miri_test::main;

tests/pass/imported_main.rs

@@ -1,5 +1,3 @@
-#![feature(imported_main)]
-
 pub mod foo {
     pub fn mymain() {
         println!("Hello, world!");

tests/pass/main_fn.rs

@@ -1,5 +1,3 @@
-#![feature(imported_main)]
-
 mod foo {
     pub(crate) fn bar() {}
 }