feat: introduce Expressive Code for code block rendering (#476)

* feat: improve code block feature with Expressive Code

* docs: add example Markdown file for visual review

* feat: improve styles of Expressive Code

* fix: errors from the merging

* chore: formatting

* chore: update config

* chore: update dependencies

* fix: fix build error

* fix: minor fixes

* fix: style tweaks

* docs: update example post

* fix: isolate copy button timeouts to avoid cross-button interference

---------

Co-authored-by: Hasenpfote <Hasenpfote36@gmail.com>

src/styles/variables.styl

@@ -56,7 +56,8 @@ define({
   --inline-code-color: var(--btn-content)
   --selection-bg: oklch(0.90 0.05 var(--hue)) oklch(0.40 0.08 var(--hue))
   --codeblock-selection: oklch(0.40 0.08 var(--hue))
-  --codeblock-bg: oklch(0.2 0.015 var(--hue)) oklch(0.17 0.015 var(--hue))
+  --codeblock-bg: oklch(0.17 0.015 var(--hue)) oklch(0.17 0.015 var(--hue))
+  --codeblock-topbar-bg: oklch(0.3 0.02 var(--hue)) oklch(0.12 0.015 var(--hue))
 
   --license-block-bg: black(0.03) var(--codeblock-bg)