fix: add missing file extension in import

[spalladino]

The missing file extension breaks the package for consumers using node16 module resolution with the following error:

node_modules/@chainsafe/libp2p-noise/dist/src/nonce.d.ts:1:36 - error TS2835: Relative import paths need explicit file extensions in EcmaScript imports when '--moduleResolution' is 'node16' or 'nodenext'. Did you mean './@types/basic.js'?

These errors are easy to miss when using node module resolution in the library. I can set up another PR to update the module resolution to node16 so these are easier to catch, if that's ok.

[mpetrunic]

Thnx!