Default to font-display: swap; closes KaTeX#2242

Google fonts seem to default to swap, and mixing them with katex makes it so that katex is invisible briefly on page load, unlike google fonts which make use of fallbacks while loading. This would unify their behavior.

Ideally there'd be an option for this, and a ?display=swap in the cdn url (like google fonts: https://developers.google.com/fonts/docs/css2#use_font-display), but I don't know if jsdelivr supports that!

src/fonts.less

@@ -2,6 +2,7 @@
 @use-ttf: true;
 @use-woff: true;
 @use-woff2: true;
+@font-display: swap;
 
 .use-woff2(@family, @family-suffix) when (@use-woff2 = true) {
     src+: url('@{font-folder}/KaTeX_@{family}-@{family-suffix}.woff2') format('woff2')
@@ -37,6 +38,7 @@
         .use-ttf(@family, @suffix);
         font-weight: @weight;
         font-style: @style;
+        font-display: @font-display;
     }
 }