fix: export types for module resolution node next

[tobeycodes]

This fixes an issue where in a monorepo using moduleResolution nodenext.

https://www.typescriptlang.org/tsconfig/#moduleResolution

src/handlers.ts:1:27 - error TS7016: Could not find a declaration file for module 'webext-bridge/background'. '/.../node_modules/.pnpm/webext-bridge@6.0.1/node_modules/webext-bridge/dist/background.cjs' implicitly has an 'any' type.
  Try `npm i --save-dev @types/webext-bridge` if it exists or add a new declaration (.d.ts) file containing `declare module 'webext-bridge/background';`

1 import { onMessage } from 'webext-bridge/background'
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