Two new fields added to the "ProductCategory" type.

[kidunot89]

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What does this implement/fix? Explain your changes.

• Adds the display and order fields to the ProductCategory type.
• Adds the ProductCategoryDisplay enumeration.
• New ProductCategory test added

NOTE: by default product category are always assigned an order

Does this close any currently open issues?

Fixes #277

Any relevant logs, error output, GraphiQL screenshots, etc?

(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Any other comments?

…

Where has this been tested?

Operating System: Ubuntu 20.04

WordPress Version: 5.4.2

[PlusA2M]

I think the field order should be menuOrder to keep consistent the naming convention along with the menuOrder of menuItems and other objects. @kidunot89