feat(gatsby-source-shopify): Add support for custom domains

[miracle2k]

Description

Fixes #19055 and allows the shopify plugin to work with shops that run on a custom domain.

See the ticket for a discussion of the issue.

[pvdz]

Please update to use includes and test both cases. Thank you :)

[pvdz]

Can you elaborate on why this is a breaking change? I'm assuming shopify domains that are not custom cannot have dots in their name?

[miracle2k]

As per the discussion in the ticket (dots not allowed in the shop name) I don't think this is a breaking change. @monizb added the label.

[monizb]

Hey!
Really sorry for the wrong label, must have mis-clicked it :)

[pvdz]

Awesome. Thanks!

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