implement @expect builtin (#19658)

* implement `@expect`

* add docs

* add a second arg for expected bool

* fix typo

* move `expect` to use BinOp

* update to newer langref format

src/Module.zig

@@ -5546,6 +5546,7 @@ pub const Feature = enum {
     /// to generate better machine code in the backends. All backends should migrate to
     /// enabling this feature.
     safety_checked_instructions,
+    can_expect,
 };
 
 pub fn backendSupportsFeature(zcu: Module, feature: Feature) bool {