refactor(core): rename ngDirectiveDef to ɵdir (#33110)

Directive defs are not considered public API, so the property
that contains them should be prefixed with Angular's marker
for "private" ('ɵ') to discourage apps from relying on def
APIs directly.

This commit adds the prefix and shortens the name from
ngDirectiveDef to dir. This is because property names
cannot be minified by Uglify without turning on property
mangling (which most apps have turned off) and are thus
size-sensitive.

Note that the other "defs" (ngFactoryDef, etc) will be
prefixed and shortened in follow-up PRs, in an attempt to
limit how large and conflict-y this change is.

PR Close #33110

packages/compiler/src/constant_pool.ts

@@ -202,7 +202,7 @@ export class ConstantPool {
       case DefinitionKind.Component:
         return 'ɵcmp';
       case DefinitionKind.Directive:
-        return 'ngDirectiveDef';
+        return 'ɵdir';
       case DefinitionKind.Injector:
         return 'ngInjectorDef';
       case DefinitionKind.Pipe: