Reimplement and simplify Hugo's template system

See #13541 for details.

Fixes #13545
Fixes #13515
Closes #7964
Closes #13365
Closes #12988
Closes #4891

tpl/tplimpl/embedded/templates/_markup/render-codeblock-goat.html

@@ -0,0 +1,18 @@
+{{ $width := .Attributes.width }}
+{{ $height := .Attributes.height }}
+{{ $class := .Attributes.class | default "" }}
+<div class="goat svg-container {{ $class }}">
+  {{ with diagrams.Goat .Inner }}
+    <svg
+      xmlns="http://www.w3.org/2000/svg"
+      font-family="Menlo,Lucida Console,monospace"
+      {{ if or $width $height }}
+        {{ with $width }}width="{{ . }}"{{ end }}
+        {{ with $height }}height="{{ . }}"{{ end }}
+      {{ else }}
+        viewBox="0 0 {{ .Width }} {{ .Height }}"
+      {{ end }}>
+      {{ .Inner }}
+    </svg>
+  {{ end }}
+</div>