proposal: make table headers sticky by default

[komuw]

• https://btxx.org/posts/Please_Make_Your_Table_Headings_Sticky/

position: sticky;
top: 0;

[ChristopherBilg]

I'm hesitant to make this the default behavior as I am unsure if an MVP really calls for it. I am however open to it if you can create a PR with the changes and possibly an example in the mvp.html file.

[michealp-coder]

My unsolicited comments: this seems like an appropriate user override (not part of the core MVP.css functionality).