fix: rolldown support - stop importing from rollup in the published types

[birkskyum]

closes #219

Fix

Replaces hard imports of Plugin, OutputOptions, OutputChunk, and GetModuleInfo from "rollup" in plugin/{index,sourcemap,data}.ts with structural stubs in a new plugin/rollup-types.ts. The stubs are subsets of both rollup's and rolldown's types, so visualizer()'s return value still drops cleanly into either bundler's plugins array.

generateBundle is typed => void | Promise<void> (rollup's signature; also accepted by rolldown's stricter => void). Internal types (OutputBundle, NormalizedOutputOptions) keep their import type from "rollup" since they don't leak into the emitted .d.ts.

Test plan

Scenario	Before	After
rolldown only, no rollup	❌ TS2307	✅ tsc --noEmit exits 0
rollup only, no rolldown	✅ exits 0	✅ exits 0 (no regression)
both installed	✅ exits 0	✅ exits 0
npm test	✅ unchanged	✅ 125/125 pass
npm run lint	✅ unchanged	✅ clean
grep 'from "rollup"' dist/	❌ hits in .d.ts	✅ none