Get wallet version from package.json

[obiyankenobi]

When we release a new wallet, we have to change the version in 3 files: package.json, public/electron.js and src/constants.js. We should try to have it only on package.json.

At least in public/electron.js, this is possible and we already do it for Sentry there:

Sentry.init({
  dsn: constants.SENTRY_DSN,
  release: process.env.npm_package_version
})

For files under src/, we might need more steps: https://stackoverflow.com/a/58517156