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]