Fix shiki shim: export stubs instead of throwing

The throwing shim caused hasShikiModule() to return false, but
import('shiki') still resolved in some code paths where the module
was imported directly without the has-check, causing undefined
destructuring and the 'e.find' TypeError.

src/shims/shiki.ts

@@ -1 +1,5 @@
-throw new Error('shiki is stubbed out to reduce bundle size')
+export const bundledThemesInfo: never[] = []
+export const bundledLanguagesInfo: never[] = []
+export function createHighlighter() {
+  return { getLoadedLanguages: () => [], getLoadedThemes: () => [], codeToTokens: () => ({ tokens: [] }), loadLanguage: async () => {}, loadTheme: async () => {}, dispose: () => {} }
+}