fix: build root shims export

[mojobeeping]

Summary

Restores the published vite-plugin-node-polyfills/shims export by building the aggregate shims entrypoint.

package.json currently exports ./shims to ./shims/dist/index.js and ./shims/dist/index.cjs, but the build only emits buffer, global, and process shims. A clean install of vite-plugin-node-polyfills@0.28.0 fails when importing the advertised root shims subpath:

ERR_MODULE_NOT_FOUND Cannot find module '.../node_modules/vite-plugin-node-polyfills/shims/dist/index.js'

This PR:

• adds shims/index.ts as the aggregate shim entrypoint
• adds build:shims:index so pnpm run build:shims emits shims/dist/index.js and shims/dist/index.cjs

Verification

pnpm run build
pnpm run test:unit
pnpm exec eslint shims/index.ts package.json
git diff --check

Packed tarball clean-consumer proof:

PUBLISHED_IMPORT_FAIL ERR_MODULE_NOT_FOUND Cannot find module '.../vite-plugin-node-polyfills/shims/dist/index.js'
PATCHED_ESM_OK function object object Blob,BlobOptions,Buffer,File,FileOptions,INSPECT_MAX_BYTES,SlowBuffer,TranscodeEncoding
PATCHED_CJS_OK function object object Blob,BlobOptions,Buffer,File,FileOptions,INSPECT_MAX_BYTES,SlowBuffer,TranscodeEncoding
PACK_HAS_ROOT_SHIMS true true true