Add webp to processable formats (#540)

* Add webp format only if using Hugo extended

This keeps PaperMod from having a hard dependency on Hugo extended.

Author: danielfdickinson

layouts/partials/cover.html

@@ -9,7 +9,10 @@
         {{- if $addLink }}<a href="{{ (path.Join .RelPermalink .Params.cover.image) | absURL }}" target="_blank"
             rel="noopener noreferrer">{{ end -}}
         {{- $sizes := (slice "360" "480" "720" "1080" "1500") }}
-        {{- $processableFormats := (slice "jpg" "jpeg" "png" "tif" "bmp" "gif") }}
+        {{- $processableFormats := (slice "jpg" "jpeg" "png" "tif" "bmp" "gif") -}}
+        {{- if hugo.IsExtended -}}
+            {{- $processableFormats = $processableFormats | append "webp" -}}
+        {{- end -}}
         {{- $prod := (hugo.IsProduction | or (eq .Site.Params.env "production")) }}
         {{- if (and (in $processableFormats $cover.MediaType.SubType) ($responsiveImages) (eq $prod true)) }}
         <img loading="lazy" srcset="{{- range $size := $sizes -}}