Optimize frontend resources for performance

[adamwoodnz]

There are a lot of unused style resources being loaded on the homepage which are affecting performance, and are highlighted in the LCP metric of pagespeed/lighthouse tests:

This likely affects other pages too. Work has been done in Showcase to stop loading these resources, and we can follow this approach in this theme.

We could consider a mu-plugin solution which stops certain resources being loaded across the network, on a page-by-page basis.

[adamwoodnz]

Noting that dashicons CSS is required for some pages like /about/stats, and had to be explicitly loaded with WP 6.5, see #354

[ryelle]

I've pulled this & the corresponding draft PR out of the "redesign" board since there hasn't been activity on it.