fix(esm-shim): match typings to actual exports

[greymoth-jp]

The hand-written types/index.d.ts for @rollup/plugin-esm-shim does not match what the package actually exports.

src/index.ts exposes the plugin as both a named and a default export, and re-exports the Output type:

export * from './types';   // Output
export default esmShim;
export { esmShim };

The typings only declare the default, under the stale name commonjsShim, and omit the named esmShim export. As a result import { esmShim } from '@rollup/plugin-esm-shim' does not type-check even though it works at runtime.

This is the same typings/exports drift that #1985 fixed for @rollup/plugin-swc. Every other plugin with a named export already declares it in its typings (swc, babel, wasm, node-resolve, html, dynamic-import-vars); esm-shim is the one that was left out.

Changes:

• declare the named esmShim export and give the default its real name, matching the shape fix(swc): fix typings to match actual exports #1985 landed (export function swc(...): Plugin; export default swc;)
• export the Output type, which src/index.ts already re-exports from ./types
• assert the named export in the package's existing test:ts type test

pnpm --filter @rollup/plugin-esm-shim test:ts reports has no exported member 'esmShim' on the current typings and passes with this change.