Fix MatchingTranslations check for Accounts translations

[jamespeilow]

What are you adding in this PR?

Fixes: https://github.com/Shopify/theme-tools/issues/930

Update MatchingTranslations theme check to prevent reporting issues with Customer Accounts translations.

• Extends the isShopifyPath check in the matching translations check so that issues are not reported if the locale path starts with shopify. or .customer_accounts.
• Adds tests

Before you deploy

• [ ] I included a patch bump changeset

[jamespeilow]

I have signed the CLA!