fix: add package main file with imports for history and live

[kinnarr]

package.json defines src/metricq.js as main file, when using metricq-js as node package. This PR adds this missing file

[kinnarr]

For webpack 4 support the main field with an existing file is required. Possible imports for live and history are <package>/src/metricq-live or <package>/src/metricq-history

Webpack 5 supports the exports field, so the possible imports for live and history are <package>/metricq-live and <package>/metricq-live. The part after the slash is customisable, for example <package>/live for metricq-live.js

[bmario]

Please update the example files with the new import