fix: dict type page parameters fall back to site parameters using merge behavior (#670)

> The `Param` method on a `Page` object looks for the given `KEY` in page parameters, and returns the corresponding value. If it cannot find the `KEY `in page parameters, it looks for the `KEY` in site parameters. If it cannot find the `KEY` in either location, the Param method returns `nil`.
>
> See: https://gohugo.io/methods/page/param/

So `.Page.Param` is only suitable for simple types.

layouts/_partials/plugin/code-block-wrapper.html

@@ -1,5 +1,5 @@
 {{- /* Hook for code blocks rendering */ -}}
-{{- $config := .Attributes | merge (.Page.Param "codeblock") -}}
+{{- $config := .Attributes | merge .Page.Params.codeblock | merge .Page.Site.Params.codeblock -}}
 {{- $result := transform.HighlightCodeBlock . -}}
 {{- $wrapper := $result.Wrapped -}}