feat(gatsby-plugin-manifest): add option to remove the "theme color" meta tag

[ivorpad]

This fixes #9977

Added an option to remove the "theme color" meta tag from the head.

• theme_color_in_head is optional, so if it's not set then the meta tag is added.
• if theme_color_in_head is set to false, then the meta tag is not added
• if theme_color_in_head is set to true, then the meta tag is added

[pieh]

We would also want to add the option in here - https://github.com/gatsbyjs/gatsby/blob/444b7274a83ac675e19350b81f63671fb928ccc4/packages/gatsby-plugin-manifest/src/__tests__/gatsby-node.js#L70-L74 to make sure that this field doesn't get added to web manifest file

[ivorpad]

Thanks a lot, Michal. Already pushed the changes. 💪

[pieh]

I think we just miss some documentation updates for it to be merged, so people are aware of that option

[ivorpad]

Done, @pieh. If there's anything wrong I'd be more than happy to give it another round.

[pieh]

I'll move the documentation part to actual plugin and will merge this

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