Add model-viewer block for the Web Components docs page

[bdelacretaz]

Add model-viewer block.

https://main--helix-website--adobe.aem.page/developer/web-components

vs

https://main--helix-website--bdelacretaz.hlx.page/developer/web-components

Description

This adds a model-viewer block for the upcoming Web Components docs page, along with the example 3D viewer component from https://modelviewer.dev/ and a <lazy-loader> Web Component to load all of that in the delayed phase.

Related Issue

#457

How Has This Been Tested?

Besides visual testing, the new code is covered by automated tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA (I'm an Adobe employee)
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[aem-code-sync]

Page	Scores	Audits	Google
/developer/web-components

[dylandepass]

LGMT

[dylandepass]

nit: extra </div>

[kptdobe]

I think the web component sample we use should be a project specific element, not something completely unrelated for which the code does not belong to the current project code.

[bdelacretaz]

the web component sample we use should be a project specific element...

I chatted with kptdobe about this, the idea is to find a Web Component that is actually useful for this website, and can serve as an example while providing useful functionality for the website. I'll look for something suitable.

[bdelacretaz]

I'll close this PR and create another one with a web component that does something useful for the website.