fix CJS default export in types

[FabianFrank]

fixes #71

[FabianFrank]

Declaring a namespace and merging the interface onto it is what prevents this from making a backwards breaking change, i.e. import build, { OnUnknown } from "pino-abstract-transport" still works.

[FabianFrank]

FYI the npm install error in CI / Test (14, windows-latest) appears unrelated to my changes:

npm ERR! code EPERM
npm ERR! syscall rename
npm ERR! path D:\a\pino-abstract-transport\pino-abstract-transport\node_modules\tap\node_modules\@jridgewell\.trace-mapping.DELETE\node_modules\@jridgewell
npm ERR! dest D:\a\pino-abstract-transport\pino-abstract-transport\node_modules\tap\node_modules\@jridgewell\trace-mapping\node_modules\@jridgewell
npm ERR! errno -4048

Maybe just requires a re-run?

[mcollina]

Could you add an exclude in the actions for windows/node v14? It's broken in gha beyond repair.

[mcollina]

lgtm