[Plugin] Listens to workbook theme color changes

[alisonjlai]

• Adds useThemeColors() hook for automatic theme color updates
• Adds themeColors property on plugin instance, right now it has two fields: backgroundColor and textColor. We can expand in the future.
• Will update readme when this is finalized.

Sample plugin app:

function App() {
  const themeColors = useThemeColors();

  return (
    <div
      style={{ backgroundColor: themeColors?.backgroundColor }}
    >
      <p style={{ color: themeColors?.textColor }}>
        ...
      </p>
    </div>
  );
}