feat: validate relative links fragments

Similar to https://github.com/DavidAnson/markdownlint/blob/main/doc/md051.md

Fixes #2

BREAKING CHANGE: Validate links fragments in relative links

test/utils.test.js

@@ -0,0 +1,37 @@
+const test = require('node:test')
+const assert = require('node:assert/strict')
+
+const {
+  convertHeadingToHTMLFragment,
+  getMarkdownHeadings
+} = require('../src/utils.js')
+
+test('utils', async (t) => {
+  await t.test('convertHeadingToHTMLFragment', async () => {
+    assert.strictEqual(
+      convertHeadingToHTMLFragment('Valid Fragments'),
+      '#valid-fragments'
+    )
+    assert.strictEqual(
+      convertHeadingToHTMLFragment('Valid Heading With Underscores _'),
+      '#valid-heading-with-underscores-_'
+    )
+    assert.strictEqual(
+      convertHeadingToHTMLFragment(
+        `Valid Heading With Quotes ' And Double Quotes "`
+      ),
+      '#valid-heading-with-quotes--and-double-quotes-'
+    )
+    assert.strictEqual(
+      convertHeadingToHTMLFragment('🚀 Valid Heading With Emoji'),
+      '#-valid-heading-with-emoji'
+    )
+  })
+
+  await t.test('getMarkdownHeadings', async () => {
+    assert.deepStrictEqual(
+      getMarkdownHeadings('# Hello\n\n## World\n\n## Hello, world!\n'),
+      ['Hello', 'World', 'Hello, world!']
+    )
+  })
+})