feat: add support for center-aligned alerts in admonition syntax (#640)

apps/test/content/posts/alert-and-admonition-test.md

@@ -154,3 +154,8 @@ This is the default example type admonition.
 {{< admonition quote "" false >}}
 This is the default quote type admonition. Aliases: `cite`
 {{< /admonition >}}
+
+## Special Types
+
+> [!center]
+> This paragraph is **center-aligned**.

layouts/_markup/render-blockquote-alert.html

@@ -1,6 +1,9 @@
 {{- $basicTypes := slice "note" "tip" "important" "warning" "caution" -}}
 
-{{- if .AlertTitle | or .AlertSign | or (.AlertType | in $basicTypes | not) -}}
+{{- if eq .AlertType "center" -}}
+  {{- /* The "center" type is a special case for centering content. */ -}}
+  <blockquote class="blockquote-center">{{ .Text }}</blockquote>
+{{- else if .AlertTitle | or .AlertSign | or (.AlertType | in $basicTypes | not) -}}
   {{- /* The extended syntax is compatible with Obsidian and FixIt admonition shortcode. */ -}}
   {{- $openMap := dict "+" true "-" false -}}
   {{- $open := index $openMap .AlertSign | default true -}}