Add a "SetContent" function

[jokogarcia]

This pull request introduces a new setContent method to the tex library, allowing users to programmatically set the content of the editor. It updates the core library, documentation, and examples to reflect this new functionality.

Core Library Changes:

• Added a setContent method to the tex object, allowing users to set the content of the editor via JavaScript. This method updates both the visible editor content and the underlying HTML content. (src/tex.js, [1] [2]

Documentation Updates:

• Updated the README.md to include setContent in the list of exported methods and added an example usage of setContent under a new "Set Content" section. (README.md, [1] [2]

Example Updates:

• Modified examples/index.html by adding a new editor instance (editor4) and demonstrating the use of the setContent method to populate it with programmatically defined content. (examples/index.html, [1] [2]

[marcellov7]

✅ LGTM — approving.

setContent is consistent with the existing getContent pattern, exported correctly (tex object + named export), and documented with a working example. It correctly syncs both the visible .tex-content and the hidden .htmlContent view.

Minor, non-blocking notes for a follow-up:

• el.value = content is a no-op for <div>-based editors (only meaningful for <textarea> bases) — harmless.
• The method doesn't fire the onChange callback, so listeners aren't notified of programmatic changes. Would be nice to plumb that through.
• README example string is missing the closing </p>.