feat: add support for Google Fonts API v2

See #1767
Close #1714

layouts/partials/site_head.html

@@ -114,10 +114,17 @@
 
   {{ end }}
 
-  {{/* We cannot use SRI with Google Fonts because the CSS is dynamically generated according to the user agent. */}}
-  {{/* Hugo's htmlEscape cannot escape "|" in Google Font URIs so we implement our own escape functionality. */}}
+  {{/* Load Google Fonts if the site's Font Theme uses them. */}}
+  {{/* Note: we cannot use SRI with Google Fonts because the CSS is dynamically generated according to the user agent. */}}
   {{ with ($scr.Get "google_fonts") }}
-  <link rel="stylesheet" {{ printf "href=\"https://fonts.googleapis.com/css?family=%s&display=swap\"" . | replaceRE "\\|" "%7C" | safeHTMLAttr }}>
+    {{ if hasPrefix . "family=" }}
+      {{/* If `google_fonts` starts with "family=", use API v2 (https://developers.google.com/fonts/docs/css2) */}}
+      <link rel="stylesheet" {{ printf "href=\"https://fonts.googleapis.com/css2?%s&display=swap\"" . | safeHTMLAttr }}>
+    {{ else }}
+      {{/* Otherwise, use API v1 */}}
+      {{/* Hugo's htmlEscape cannot escape "|" in Google Font URIs so we implement our own escape functionality. */}}
+      <link rel="stylesheet" {{ printf "href=\"https://fonts.googleapis.com/css?family=%s&display=swap\"" . | replaceRE "\\|" "%7C" | safeHTMLAttr }}>
+    {{ end }}
   {{ end }}
 
   {{ $css_comment := printf "/*!* Source Themes Academic v%s (https://sourcethemes.com/academic/) */\n" site.Data.academic.version }}