Add generic font fallbacks & prune some unused styles

[moritz89]

From: jagenjo/litegraph.js#294

• added generic fallback (sans-serif) to every font specification
• enclosed all explicit font names in quotes for consistency and visual difference
• h1 and h2 elements don't seem to be used anywhere outside of the docs, so I pruned them from litegraph-editor.css and editor/style.css

This makes the UI look better on Linux which doesn't have fonts like Tahoma and Verdana. Personally, I would go further and re-do all of the font assignment based on Boostrap 5's system font stack, to quote, "for optimum text rendering on every device and OS".
But it's be up to the maintainer to say whether this is desirable :)

@atlasan might want to take a look at this as well, since I see they've been editing the CSS