Setting to use old versions of Mavo

[LeaVerou]

I was trying to figure out what caused this regression and I realized this would be incredibly useful: A setting to use an older version of Mavo in the tests.

Not sure yet if this can be generalized into an htest feature, or if we should implement it here. I suspect it's simpler to implement it here and figure out if/how to generalize later.

An easy but dirty way to implement this is to use a URL param (?version=v0.2.0) and document.write() to output a <script> element with the correct URL. The downside is we'd need to do this on every single tests page. Maybe it's time to move htest to 11ty?

[LeaVerou]

Pushed a prototype in https://test.mavo.io/property.html which seems to work well:

• https://test.mavo.io/property.html?version=stable
• https://test.mavo.io/property.html?version=v0.2
• https://test.mavo.io/property.html?version=v0.2.3
• https://test.mavo.io/property.html?version=v0.1.6

However, this is a nontrivial code snippet, so we should find a way to reuse, rather than having to maintain it across 24 test sections.
We can include it in a <script> that we link to, the downside being slower loading.