Stricter html string check

[wxiaoguang]

Fix #638

/^\s*<[a-zA-Z]/.test(" \n\t<a") => true

The real world case is like this:

const s = `
  <div ...>
   ...
  </div>
`;

[Copilot]

Pull Request Overview

This PR ensures that HTML literal strings correctly handle leading whitespace by updating the regex match pattern.

• Updated regex in unescaped-html-literal.js to allow for preceding whitespace before an HTML tag.
• Fixes potential false negatives in matching real-world HTML snippets.

Comments suppressed due to low confidence (1)

lib/rules/unescaped-html-literal.js:18

• Consider adding a comment explaining that the updated regex now supports leading whitespace before an HTML tag to aid future maintainability.

    const htmlOpenTag = /^\s*<[a-zA-Z]/