Improve theme performance

[jsnmrs]

• Switches to variable font version of Inter
• Preloads font file
• Add explicit width and height to images
• Add loading="lazy" to images
• Compresses SCSS output
• Removes HTML compression template (https://github.com/penibelst/jekyll-compress-html appears to be no longer updated)

@katydecorah could you take a look at this, locally, for any visible changes? In theory, this shouldn't yield a visible difference, besides the contents of the variable table. Thanks!