Improve SafeString type tests, include in preview types

Building on the previous, beta-only PR, update the type tests to check
for the constructibility of the type, and make sure it is included in
the types users can actually consume. (This is the part we will end up
backporting to LTS!)

type-tests/@ember/template-tests.ts

@@ -1,18 +1,17 @@
-import { SafeString } from '@ember/template/-private/handlebars';
-import { htmlSafe, isHTMLSafe } from '@ember/template';
+import { htmlSafe, isHTMLSafe, SafeString } from '@ember/template';
 import { expectTypeOf } from 'expect-type';
 
-const handlebarsSafeString: SafeString = htmlSafe('lorem ipsum...');
-expectTypeOf(htmlSafe('lorem ipsum...')).toEqualTypeOf<SafeString>();
-// @ts-expect-error
-const regularString: string = htmlSafe('lorem ipsum...');
-
+let trusted = htmlSafe('lorem ipsum...');
+expectTypeOf(trusted).toEqualTypeOf<SafeString>();
+expectTypeOf(trusted).not.toBeString();
 expectTypeOf(isHTMLSafe).guards.toEqualTypeOf<SafeString>();
+expectTypeOf(trusted.toHTML()).toBeString();
+expectTypeOf(trusted.toString()).toBeString();
 
-function isSafeTest(a: string | SafeString) {
-  if (isHTMLSafe(a)) {
-    a = a.toString();
-  }
+expectTypeOf<SafeString>().toMatchTypeOf<{
+  toString(): string;
+  toHTML(): string;
+}>();
 
-  a.toLowerCase();
-}
+// @ts-expect-error -- we do not allow construction by exporting only the type.
+new SafeString('whatever');

types/preview/@ember/template/index.d.ts

@@ -1,5 +1,6 @@
 declare module '@ember/template' {
   import { SafeString } from '@ember/template/-private/handlebars';
+  export type { SafeString };
   export function htmlSafe(str: string): SafeString;
   export function isHTMLSafe(str: unknown): str is SafeString;
 }