chore(prlint): enforce lowercase pr title (aws#29002)

Enforces that the title looks like `chore(prlint): enforce lowercase pr title` and not `chore(prlint): Enforce lowercase pr title`. 

Why does this matter? It doesn't really. But [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/) tells us to be consistent, and their examples are all in lowercase, and most of our pr titles are already lowercase.

----

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

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

@@ -542,20 +542,6 @@ export class PullRequestLinter {
       testRuleSet: [{ test: featureContainsIntegTest }, { test: fixContainsIntegTest }],
     });
 
-    validationCollector.validateRuleSet({
-      testRuleSet: [{ test: validateBreakingChangeFormat }],
-    });
-
-    validationCollector.validateRuleSet({
-      testRuleSet: [{ test: validateTitlePrefix }],
-    });
-    validationCollector.validateRuleSet({
-      testRuleSet: [{ test: validateTitleScope }],
-    });
-    validationCollector.validateRuleSet({
-      testRuleSet: [{ test: validateBranch }],
-    })
-
     validationCollector.validateRuleSet({
       exemption: shouldExemptBreakingChange,
       exemptionMessage: `Not validating breaking changes since the PR is labeled with '${Exemption.BREAKING_CHANGE}'`,
@@ -570,7 +556,17 @@ export class PullRequestLinter {
     validationCollector.validateRuleSet({
       exemption: (pr) => pr.user?.login === 'aws-cdk-automation',
       testRuleSet: [{ test: noMetadataChanges }],
-    })
+    });
+
+    validationCollector.validateRuleSet({
+      testRuleSet: [
+        { test: validateBreakingChangeFormat },
+        { test: validateTitlePrefix },
+        { test: validateTitleScope },
+        { test: validateTitleLowercase },
+        { test: validateBranch },
+      ],
+    });
 
     await this.deletePRLinterComment();
     try {
@@ -735,6 +731,18 @@ function validateTitleScope(pr: GitHubPr): TestResult {
   return result;
 }
 
+function validateTitleLowercase(pr: GitHubPr): TestResult {
+  const result = new TestResult();
+  const start = pr.title.indexOf(':');
+  const firstLetter = pr.title.charAt(start + 2);
+  console.log(firstLetter, firstLetter.toLocaleLowerCase());
+  result.assessFailure(
+    firstLetter !== firstLetter.toLocaleLowerCase(),
+    'The first word of the pull request title should not be capitalized. If the title starts with a CDK construct, it should be in backticks "``".',
+  );
+  return result;
+}
+
 /**
  * Check that the PR is not opened from main branch of author's fork
  *

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

@@ -197,6 +197,34 @@ describe('commit message format', () => {
     const prLinter = configureMock(issue, undefined);
     expect(await prLinter.validatePullRequestTarget(SHA)).resolves;
   });
+
+  test('invalid capitalized title', 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);
+    await expect(prLinter.validatePullRequestTarget(SHA)).rejects.toThrow(/The first word of the pull request title should not be capitalized. If the title starts with a CDK construct, it should be in backticks "``"/);
+  });
+
+  test('valid capitalized title with backticks', async () => {
+    const issue = {
+      number: 1,
+      title: 'fix(aws-cdk-lib): `CfnConstruct`',
+      body: '',
+      labels: [{ name: 'pr-linter/exempt-test' }, { name: 'pr-linter/exempt-integ-test' }],
+      user: {
+        login: 'author',
+      },
+    };
+    const prLinter = configureMock(issue, undefined);
+    expect(await prLinter.validatePullRequestTarget(SHA)).resolves;
+  });
 });
 
 describe('ban breaking changes in stable modules', () => {
@@ -1018,11 +1046,11 @@ describe('integration tests required on features', () => {
 
     test('with aws-cdk-automation author', async () => {
       const pr = {
-        title: 'chore: Update regions',
+        title: 'chore: update regions',
         number: 1234,
         labels: [],
         user: {
-          login: 'aws-cdk-automation'
+          login: 'aws-cdk-automation',
         },
       };
 
@@ -1032,7 +1060,7 @@ describe('integration tests required on features', () => {
 
     test('with another author', async () => {
       const pr = {
-        title: 'chore: Update regions',
+        title: 'chore: update regions',
         number: 1234,
         labels: [],
         user: {