fix: stop requiring shiki for type-checking streamdown

[ghostyfreak]

Description

streamdown re-exported BundledLanguage, BundledTheme, and ThemeRegistrationAny from shiki in its published type declarations, but shiki was only a devDependency. Consumers who install streamdown and run tsc (especially with skipLibCheck: false) hit:

Cannot find module 'shiki' or its corresponding type declarations.

even when they never use Shiki directly. Runtime highlighting already lives in @streamdown/code (which correctly depends on shiki); core streamdown only needed those types for its plugin API.

This PR defines local structural types for the re-exports so published .d.ts files no longer import from shiki.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Related Issues

Fixes #562

Changes Made

• Add lib/shiki-types.ts with local BundledLanguage, BundledTheme, and ThemeRegistrationAny types
• Re-export those types from plugin-types / package entry instead of from "shiki"
• Remove unused shiki devDependency from streamdown
• Use method syntax on CodeHighlighterPlugin so plugins from @streamdown/code (narrower Shiki language/theme unions) remain assignable
• Add changeset (patch)

Testing

• All existing tests pass
• Manually tested the changes

Verification

1. Unit tests: pnpm --filter streamdown test — 982 tests passed
2. Build: pnpm --filter streamdown build — published dist/index.d.ts has no from 'shiki' import
3. Consumer pack test (reproduces Type declarations import from shiki, but shiki is only a devDependency #562):
   • Packed the fixed package and installed it in a fresh app without shiki
   • tsc with skipLibCheck: false no longer reports Cannot find module 'shiki'
   • Against published streamdown@2.5.0 under the same config, the shiki module errors still appear
4. Plugin assignability: packed streamdown + @streamdown/code and confirmed <Streamdown plugins={{ code }} /> type-checks

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• My changes generate no new warnings or errors
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Changeset

• I have created a changeset for these changes

Additional Notes

BundledLanguage / BundledTheme are intentionally widened to string on the core package surface. Precise Shiki unions remain available from @streamdown/code / shiki for apps that install the highlighter plugin. This avoids pulling the large shiki package into every streamdown consumer solely for types.

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