Add structured data testing

[fulldecent]

Add structured-data-testing-tool

See example https://github.com/ThatGuySam/doesitarm/blob/6e11fbe8e79156282aa9c5d61f87808182814325/test/main.js#L8

Specifically, we want to ensure that any json+ld scripts in build/**/*.html will have the correct schema formats.

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Before we start this, do a simple test across all our pages with like

find | exec yarn dlx structured-data-testing-tool test...

If there are some errors on any built page on our sites then setting up this testing will be a win

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End result:

• Add script to package.json to validate after build
• Add it to our CI testing pipeline after build
• Create test cases with expected failures and include that to CI build (see existing CI build and fixtures for to check HTML Validate rules are working)