Inject /node_modules/.bin into PATH to mimic npm/pnpm/yarn scripts behaviour#35

At the moment, if `spawn` scripts are referencing locally installed npm executables, they are going to be not found. ```ts spawn`tsx` ``` will produce "tsx program not found" Users can workaround this by prefixing them with `pnpm` (or whatever package manager), e.g. ```ts spawn`pnpm tsx` ``` However, we should consider matching `pnpm` behaviour and just inject the appropriate path.