perf(rehype-shiki): give the default themes by name

The default themes were passed as theme objects, which Shiki
normalizes, and parses into a new vscode-textmate theme, on every
highlight, and whose tokens can't be cached for both. They're
registered with the highlighter anyway, so they're now given by name,
in a `DEFAULT_THEME_OPTIONS` constant.

The output is the same. Given by name, a theme's own color replacements
no longer apply to the code block's foreground and background, but the
default themes' only replacement (`#616e88`) is neither.

Assisted-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: ovflowd

.changeset/rehype-shiki-tokenization-cache.md

@@ -0,0 +1,5 @@
+---
+'@node-core/rehype-shiki': minor
+---
+
+Cache each line's tokens for all the themes code is highlighted in when two or more are given by name, as the default themes now are, instead of tokenizing it again for each. The new `@node-core/rehype-shiki/tokenization-cache` module exports `withTokenizationCache`, which does this for any Shiki highlighter.

packages/rehype-shiki/src/__tests__/highlighter.test.mjs

@@ -87,7 +87,10 @@ describe('createHighlighter', async () => {
       highlighter.highlightToHtml('const x = 1;', 'javascript');
 
       const [, options] = mockShiki.codeToHtml.mock.calls.at(-1).arguments;
-      assert.deepStrictEqual(Object.keys(options.themes), ['light', 'dark']);
+      assert.deepStrictEqual(options.themes, {
+        light: 'github-light-default',
+        dark: 'nord',
+      });
       assert.strictEqual(options.defaultColor, 'light');
     });
   });

packages/rehype-shiki/src/highlighter.mjs

@@ -20,6 +20,15 @@ const DEFAULT_THEMES = {
   dark: DEFAULT_DARK_THEME,
 };
 
+// The default themes by name: Shiki prepares a theme object again for every
+// highlight, and only caches tokens for all the themes given by name
+const DEFAULT_THEME_OPTIONS = {
+  themes: Object.fromEntries(
+    Object.entries(DEFAULT_THEMES).map(([color, { name }]) => [color, name])
+  ),
+  defaultColor: 'light',
+};
+
 const FALLBACK_LANGUAGE = 'text';
 
 /**
@@ -64,7 +73,7 @@ const createHighlighter = ({ coreOptions = {}, highlighterOptions = {} }) => {
     ? {}
     : coreOptions.themes
       ? { theme: options.themes[0] }
-      : { themes: DEFAULT_THEMES, defaultColor: 'light' };
+      : DEFAULT_THEME_OPTIONS;
 
   const loadedLanguages = new Set(
     shiki.getLoadedLanguages().map(lang => lang.toLowerCase())