Add media property to MetaTag interface

[kaje94]

Is your feature request related to a problem? Please describe.
I'm trying to provide a theme-color meta tag for both dark theme as well as light theme. The standard themeColor prop only works with light theme.

Describe the solution you'd like
Add a media property to MetaTag interface so that users can define the content of the meta tag based on media property.

The meta tag added to html head would look something like this

<meta content="#FFFFFF"  name="theme-color" media="(prefers-color-scheme: dark)" />

Describe alternatives you've considered
adding darkThemeColor property to DefaultSeoProps might also work but it will only handle this particular use case

[ilyaliao]

If you use next-themes, try this.

import { useTheme } from 'next-themes';
const { resolvedTheme } = useTheme();

<NextSeo
  themeColor={resolvedTheme === 'dark' ? darkColor : lightColor}
/>