Add test to verify exported names are stable, add missing test files after earlier refactor.

test/snapshots/markdownlint-test-exports.mjs.md

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+# Snapshot report for `test/markdownlint-test-exports.mjs`
+
+The actual snapshot is saved in `markdownlint-test-exports.mjs.snap`.
+
+Generated by [AVA](https://avajs.dev).
+
+## exported names
+
+> Snapshot 1
+
+    {
+      markdownlint: [
+        'applyFix',
+        'applyFixes',
+        'getVersion',
+        'resolveModule',
+      ],
+      'markdownlint/async': [
+        'lint',
+        'readConfig',
+      ],
+      'markdownlint/helpers': [
+        'addError',
+        'addErrorContext',
+        'addErrorDetailIf',
+        'allPunctuation',
+        'allPunctuationNoQuestion',
+        'clearHtmlCommentText',
+        'cloneIfArray',
+        'cloneIfUrl',
+        'default',
+        'ellipsify',
+        'endOfLineGemojiCodeRe',
+        'endOfLineHtmlEntityRe',
+        'escapeForRegExp',
+        'expandTildePath',
+        'frontMatterHasTitle',
+        'frontMatterRe',
+        'getHtmlAttributeRe',
+        'getPreferredLineEnding',
+        'getReferenceLinkImageData',
+        'hasOverlap',
+        'inlineCommentStartRe',
+        'isBlankLine',
+        'isEmptyString',
+        'isNumber',
+        'isObject',
+        'isString',
+        'isUrl',
+        'newLineRe',
+        'nextLinesRe',
+      ],
+      'markdownlint/promise': [
+        'extendConfig',
+        'lint',
+        'readConfig',
+      ],
+      'markdownlint/style/all': [
+        'comment',
+        'default',
+      ],
+      'markdownlint/style/cirosantilli': [
+        'comment',
+        'default',
+        'MD003',
+        'MD004',
+        'MD007',
+        'MD030',
+        'MD033',
+        'MD035',
+      ],
+      'markdownlint/style/prettier': [
+        'comment',
+        'blanks-around-fences',
+        'blanks-around-headings',
+        'blanks-around-lists',
+        'code-fence-style',
+        'emphasis-style',
+        'heading-start-left',
+        'heading-style',
+        'hr-style',
+        'line-length',
+        'list-indent',
+        'list-marker-space',
+        'no-blanks-blockquote',
+        'no-hard-tabs',
+        'no-missing-space-atx',
+        'no-missing-space-closed-atx',
+        'no-multiple-blanks',
+        'no-multiple-space-atx',
+        'no-multiple-space-blockquote',
+        'no-multiple-space-closed-atx',
+        'no-trailing-spaces',
+        'ol-prefix',
+        'strong-style',
+        'ul-indent',
+      ],
+      'markdownlint/style/relaxed': [
+        'comment',
+        'default',
+        'whitespace',
+        'line_length',
+        'ul-indent',
+        'no-inline-html',
+        'no-bare-urls',
+        'fenced-code-language',
+        'first-line-h1',
+      ],
+      'markdownlint/sync': [
+        'lint',
+        'readConfig',
+      ],
+    }