Add syntax highlighting on code blocks, plus a Code Highlighting post

Colors <pre><code> blocks site-wide via vendored highlight.js v11.11.1
(pinned to that stable tag, not main, which tracks an in-progress
11.0.0-beta1), auto-detected and restricted to
configure({ languages: ['php', 'bash', 'xml', 'css', 'python',
'javascript', 'yaml', 'json', 'ini'] }) - no per-block markup needed for
the ~60 existing code blocks across the site. css/python/javascript ship
in the core bundle; yaml/json/ini (ini covers .env-style files too)
don't and are vendored as separate per-language files.

Themes swap with the existing dark/light toggle: ir-black (dark) +
github (light, resolving the entry's own open question), via the same
data-theme-driven mechanism as the main palette
(syntax-highlight-init.twig/syntax-highlight.twig, mirroring
theme-init.twig/nav.twig's split) - a MutationObserver swaps the theme
link live without touching the existing toggle button's click handler.

Twig-syntax code blocks have no highlight.js grammar and are marked
class="nohighlight" by hand (15 blocks across 9 files, found by grepping
for literal {% %}/{{ }} syntax rather than guessing) rather than
force-matched into the restricted candidate set, which would color them
wrong instead of leaving them plain.

One correction to the original backlog entry's suggested approach: it
suggested vendoring highlight.js under novaconium/vendor/ next to Twig.
That would have silently 404ed on every request - Twig is server-side
PHP, never fetched by a browser, but highlight.js's .js/.css files are,
and only public/ is web-reachable. Vendored to public/vendor/highlightjs/
instead; documented in AGENTS.md and a new upgrading-highlightjs doc,
since public/ isn't touched by the usual novaconium/-swap framework
update workflow, so a future highlight.js bump won't propagate to
existing projects automatically the way it does for everything else
under novaconium/.

Caught two real bugs via testing rather than review: hljs.highlightAll()
silently no-ops if called before the document finishes parsing rather
than deferring itself, and a bash example starting with the word "php"
auto-detects as PHP, not bash.

Also adds App/pages/blog/code-highlighting/ - a new blog post
demonstrating the feature with a verified worked example in each of the
nine languages, plus how to force a language via an explicit
language-<name> class when auto-detection isn't enough.

Closes the "Syntax highlighting on code blocks" backlog item in
novaconium/ISSUES.md.

public/vendor/highlightjs/styles/github.min.css

@@ -0,0 +1,10 @@
+pre code.hljs{display:block;overflow-x:auto;padding:1em}code.hljs{padding:3px 5px}/*!
+  Theme: GitHub
+  Description: Light theme as seen on github.com
+  Author: github.com
+  Maintainer: @Hirse
+  Updated: 2021-05-15
+
+  Outdated base version: https://github.com/primer/github-syntax-light
+  Current colors taken from GitHub's CSS
+*/.hljs{color:#24292e;background:#fff}.hljs-doctag,.hljs-keyword,.hljs-meta .hljs-keyword,.hljs-template-tag,.hljs-template-variable,.hljs-type,.hljs-variable.language_{color:#d73a49}.hljs-title,.hljs-title.class_,.hljs-title.class_.inherited__,.hljs-title.function_{color:#6f42c1}.hljs-attr,.hljs-attribute,.hljs-literal,.hljs-meta,.hljs-number,.hljs-operator,.hljs-selector-attr,.hljs-selector-class,.hljs-selector-id,.hljs-variable{color:#005cc5}.hljs-meta .hljs-string,.hljs-regexp,.hljs-string{color:#032f62}.hljs-built_in,.hljs-symbol{color:#e36209}.hljs-code,.hljs-comment,.hljs-formula{color:#6a737d}.hljs-name,.hljs-quote,.hljs-selector-pseudo,.hljs-selector-tag{color:#22863a}.hljs-subst{color:#24292e}.hljs-section{color:#005cc5;font-weight:700}.hljs-bullet{color:#735c0f}.hljs-emphasis{color:#24292e;font-style:italic}.hljs-strong{color:#24292e;font-weight:700}.hljs-addition{color:#22863a;background-color:#f0fff4}.hljs-deletion{color:#b31d28;background-color:#ffeef0}

public/vendor/highlightjs/styles/ir-black.min.css

@@ -0,0 +1 @@
+pre code.hljs{display:block;overflow-x:auto;padding:1em}code.hljs{padding:3px 5px}.hljs{background:#000;color:#f8f8f8}.hljs-comment,.hljs-meta,.hljs-quote{color:#7c7c7c}.hljs-keyword,.hljs-name,.hljs-selector-tag,.hljs-tag{color:#96cbfe}.hljs-attribute,.hljs-selector-id{color:#ffffb6}.hljs-addition,.hljs-selector-attr,.hljs-selector-pseudo,.hljs-string{color:#a8ff60}.hljs-subst{color:#daefa3}.hljs-link,.hljs-regexp{color:#e9c062}.hljs-doctag,.hljs-section,.hljs-title,.hljs-type{color:#ffffb6}.hljs-bullet,.hljs-literal,.hljs-symbol,.hljs-template-variable,.hljs-variable{color:#c6c5fe}.hljs-deletion,.hljs-number{color:#ff73fd}.hljs-emphasis{font-style:italic}.hljs-strong{font-weight:700}