Write up and down arrows on internal links to headings

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

layouts/_default/_markup/render-heading.html

@@ -0,0 +1,4 @@
+{{- .Page.Scratch.SetInMap "definedSections" (printf "#%s" .Anchor) true -}}
+<h{{ .Level }} id="{{ .Anchor }}">
+  {{- .Text | safeHTML -}}
+</h{{ .Level }}>

layouts/_default/_markup/render-link.html

@@ -1,13 +1,30 @@
+{{- $scratch := newScratch -}}
 {{- $absoluteDest := absLangURL .Destination -}}
 {{- $siteRootUrl := absLangURL "" -}}
 {{- $isExternal := not (hasPrefix $absoluteDest $siteRootUrl) -}}
+{{- $isSamePage := hasPrefix .Destination "#" -}}
+
+{{- if $isSamePage -}}
+  {{- $scratch.Set "class" "same-page-link" -}}
+  {{- if index (.Page.Scratch.Get "definedSections") .Destination -}}
+    {{- $scratch.Set "icon" "arrow-up" -}}
+  {{- else -}}
+    {{- $scratch.Set "icon" "arrow-down" -}}
+  {{- end -}}
+{{- else if $isExternal -}}
+  {{- $scratch.Set "class" "external-link" -}}
+  {{- $scratch.Set "icon" "external-link" -}}
+{{- end -}}
 <a href="{{ .Destination | safeURL }}"
   {{- with .Title }} title="{{ . }}"{{ end -}}
+  {{- with $scratch.Get "class" -}}
+    class="{{ . }}"
+  {{- end -}}
   {{- if $isExternal -}}
-   class="external-link" target="_blank" rel="noopener noreferrer"
+    target="_blank" rel="noopener noreferrer"
   {{- end -}}
 >
   {{- with .Text | safeHTML }}{{ . }}{{ end -}}
-  {{- if $isExternal -}}{{- partial "icon.html" "external-link" -}}{{- end -}}
+  {{- with $scratch.Get "icon" -}}{{- partial "icon.html" . -}}{{- end -}}
 </a>
 {{- /* chomp trailing newline */ -}}