Issue #3616088: Support code blocks (language-mermaid) in addition to the [mermaid] shortcode

Issue: https://www.drupal.org/project/mermaid/issues/3616088

The filter only recognises the [mermaid]...[/mermaid] shortcode. That is not what any editor produces: CKEditor 5's Code Block plugin emits

, and so does every Markdown converter
for a ```mermaid fence. A site using the standard editor has to hand-write
the shortcode in source view, and content pasted or generated as Markdown
never renders.

Adds the code block form alongside the shortcode. Both go through
processMermaid() and produce the same  markup, so no JS
change is needed.

Three details worth calling out for review:

1. The  element deliberately does not survive into the output. Sites
   that also run a highlight.js integration call hljs.highlightAll(), which
   walks every "pre code" in the document and auto-detects a language no
   matter what the filter order is. Leaving the  in place would get
   the diagram source syntax-highlighted underneath the rendered graph, and
   would make the highlighter request a "mermaid" grammar that highlight.js
   does not ship, producing a 404 on every page with a diagram. Replacing
   the whole  pair avoids both.

2. processMermaid() concatenated the %%{init}%% directive straight onto the
   diagram source. That happened to work for the shortcode, whose captured
   text starts with the newline after [mermaid], but it breaks for a code
   block whose content starts at the diagram keyword: the directive and the
   keyword end up on the same line and Mermaid fails to parse. The directive
   now always ends with a newline, and the source is trimmed.

3. The libraries were attached unconditionally, so every page rendering
   filtered text pulled the Mermaid bundle (around 1 MB) whether or not it
   contained a diagram. They are now attached only when a diagram was
   actually found, which is the pattern highlightjs_input_filter uses.

Verified on Drupal 10.6 with both syntaxes rendering to SVG on the same
page, a PHP code block on that page keeping its normal highlighting, no
console errors, and no library attached on pages without a diagram.

Closes #3616088