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 which keep it from tokenizing in a single pass. They're
registered with the highlighter anyway, so they're now given by name.

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/single-pass-rehype-shiki-highlighter.md

@@ -0,0 +1,5 @@
+---
+'@node-core/rehype-shiki': minor
+---
+
+Tokenize code once for all the themes it's highlighted in when two or more are given by name, as the default themes now are. The new `@node-core/rehype-shiki/single-pass` module exports `withSinglePassTokenization`, 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

@@ -64,7 +64,16 @@ const createHighlighter = ({ coreOptions = {}, highlighterOptions = {} }) => {
     ? {}
     : coreOptions.themes
       ? { theme: options.themes[0] }
-      : { themes: DEFAULT_THEMES, defaultColor: 'light' };
+      : {
+          // By name, as Shiki prepares a theme object again for every
+          // highlight, and code is only tokenized in a single pass for themes
+          // given by name
+          themes: {
+            light: DEFAULT_THEMES.light.name,
+            dark: DEFAULT_THEMES.dark.name,
+          },
+          defaultColor: 'light',
+        };
 
   const loadedLanguages = new Set(
     shiki.getLoadedLanguages().map(lang => lang.toLowerCase())