package.json: specify the files to include in npm

[XhmikosR]

Before:

npm notice === Tarball Details ===
npm notice name:          @lambdatest/node-tunnel
npm notice version:       3.0.12
npm notice filename:      lambdatest-node-tunnel-3.0.12.tgz
npm notice package size:  15.5 kB
npm notice unpacked size: 69.9 kB
npm notice shasum:        2dd916cb59b6e8e0c0aecd9b50894b94ab5e01bc
npm notice integrity:     sha512-ygOCBBuFieffu[...]eVaLU4qwITFmQ==
npm notice total files:   15

After:

npm notice === Tarball Details ===
npm notice name:          @lambdatest/node-tunnel
npm notice version:       3.0.12
npm notice filename:      lambdatest-node-tunnel-3.0.12.tgz
npm notice package size:  14.4 kB
npm notice unpacked size: 67.3 kB
npm notice shasum:        cced49466e6a7a69de202f3cc9c527ada8f7e8b1
npm notice integrity:     sha512-7Aspovb369LEm[...]J6uQ6UQfWMl9Q==
npm notice total files:   9

[XhmikosR]

Could someone merge this?