Add `GT.write_html()` as a helper function for easier HTML output

[jrycw]

Hello team,

I've noticed that some users expect an easier way to interact with our tables. While GT.as_raw_html() is great, it doesn’t fully meet those expectations. To address this, I’d like to propose adding GT.write_html() as a helper function, simplifying the process of writing the table’s HTML directly to a file without needing to use open().

I believe this is a useful addition, and if necessary, we could even rename it to GT._write_html() for unofficial use.