chore(prlint): allow 'aws-cdk-lib' as a pr title scope (#25181)

If a PR applies to aws-cdk-lib as a whole, we want to label it as such. Today, if you have a PR titled "fix(aws-cdk-lib): some change", the PR linter will include this message:

```
❌ The title of the pull request should omit 'aws-' from the name of modified packages. Use 'cdk-lib' instead of 'aws-cdk-lib'.
```

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: madeline-k

tools/@aws-cdk/prlint/lint.ts

@@ -468,14 +468,15 @@ function hasLabel(pr: GitHubPr, labelName: string): boolean {
  */
 function validateTitleScope(pr: GitHubPr): TestResult {
   const result = new TestResult();
+  const scopesExemptFromThisRule = [ 'aws-cdk-lib' ];
   // Specific commit types are handled by `validateTitlePrefix`. This just checks whether
   // the scope includes an `aws-` prefix or not.
   // Group 1: Scope with parens - "(aws-<name>)"
   // Group 2: Scope name - "aws-<name>"
   // Group 3: Preferred scope name - "<name>"
   const titleRe = /^\w+(\((aws-([\w_-]+))\))?: /;
   const m = titleRe.exec(pr.title);
-  if (m) {
+  if (m && !scopesExemptFromThisRule.includes(m[2])) {
     result.assessFailure(
       !!(m[2] && m[3]),
       `The title of the pull request should omit 'aws-' from the name of modified packages. Use '${m[3]}' instead of '${m[2]}'.`,

tools/@aws-cdk/prlint/test/lint.test.ts

@@ -127,6 +127,20 @@ describe('commit message format', () => {
     expect(await prLinter.validate()).resolves;
   });
 
+  test('valid with aws-cdk-lib as a scope', async () => {
+    const issue = {
+      number: 1,
+      title: 'fix(aws-cdk-lib): some title',
+      body: '',
+      labels: [{ name: 'pr-linter/exempt-test' }, { name: 'pr-linter/exempt-integ-test' }],
+      user: {
+        login: 'author',
+      },
+    };
+    const prLinter = configureMock(issue, undefined);
+    expect(await prLinter.validate()).resolves;
+  });
+
   test.each(['core', 'prlint', 'awslint'])('valid scope for packages that dont use aws- prefix', async (scope) => {
     const issue = {
       number: 1,
@@ -142,8 +156,8 @@ describe('commit message format', () => {
   })
 });
 
-describe.only('ban breaking changes in stable modules', () => {
-  test.only('breaking change in stable module', async () => {
+describe('ban breaking changes in stable modules', () => {
+  test('breaking change in stable module', async () => {
     const issue = {
       number: 1,
       title: 'chore(s3): some title',