rustdoc-search: search never type with !

This feature extends rustdoc to support the syntax that most users will
naturally attempt to use to search for diverging functions.
Part of #60485

It's already possible to do this search with `primitive:never`, but
that's not what the Rust language itself uses, so nobody will try it if
they aren't told or helped along.

tests/rustdoc-js/never-search.rs

@@ -0,0 +1,13 @@
+#![feature(never_type)]
+
+#[allow(nonstandard_style)]
+pub struct never;
+
+pub fn loops() -> ! { loop {} }
+pub fn returns() -> never { never }
+
+pub fn impossible(x: !) { match x {} }
+pub fn uninteresting(x: never) { match x { never => {} } }
+
+pub fn box_impossible(x: Box<!>) { match *x {} }
+pub fn box_uninteresting(x: Box<never>) { match *x { never => {} } }