Add engines to package.json

[ivmartel]

The engines text specifies the compatible node versions (see doc). This will produce an error if run in strict mode, otherwise a warning.

[ivmartel]

node v12 gives an encoding error:

internal/encoding.js:398
throw new ERR_ENCODING_NOT_SUPPORTED(encoding);
^
RangeError [ERR_ENCODING_NOT_SUPPORTED]: The "iso-8859-1" encoding is not supported
at new TextDecoder (internal/encoding.js:398:15)
at new A.dicom.DataReader (/home/yves/dev/src/github/local/node/parse/node_modules/dwv/dist/dwv.min.js:3:24376)
at A.dicom.DicomParser.parse (/home/yves/dev/src/github/local/node/parse/node_modules/dwv/dist/dwv.min.js:3:45475)

This is because v12 does not come with full internationalisation by default (see v12 class_util_textdecoder). Full include started at v13 (see v13 changelog and v13 class_util_textdecoder)

So the minimum supported node will be v14 (the next LTS after v12).