chore: validate (lint) production build HTML files

[jbmoelker]

Issue #6 rightly states, each page must only have a single h1 and headings should not skip a level.
This PR introduces lint:html which uses html-validate to check the HTML pages of the production build.
It correctly finds these issues:

> head-start@0.0.1 lint:html
> html-validate dist/


/home/runner/work/head-start/head-start/dist/en/page-1/index.html
Error:   3:774  error  Multiple <h1> are not allowed                                       heading-level
Error:   3:810  error  Heading level can only increase by one, expected <h3> but got <h4>  heading-level

✖ 2 problems (2 errors, 0 warnings)

More information:
  https://html-validate.org/rules/heading-level.html

Error: Process completed with exit code 1.

But, what do we want to do with that? Because these issues are caused by a content editor using incorrect semantics in the CMS. However failing the build and then checking the logs isn't really a viable option.

Update: now partially resolved by #81 .

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Changes

Associated issue

Part of #6

How to test

Checklist

• I have performed a self-review of my own code
• I have made sure that my PR is easy to review (not too big, includes comments)
• I have added/updated tests to prove that my feature works (if not possible please explain why)
• I have made changes to the README and if the change affects the project setup (npm commands changed, new service added, environmental variable added)
• I have added a decision log entry if the change affects the architecture or changes a significant technology
• I have notified a reviewer

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[jbmoelker]

This PR would be a good extension to #58 from which it reuses part of the setup.

[jbmoelker]

Regression on package-lock.json issues. Closing in favour of #100 .