feat: getVersion with version from package.json

[shevnv]

Fixes #971

import {getVersion} from 'zx'

const [major] = (getVersion() || '').split('.').map(Number)
if (major < 6)
  throw new Error('zx >= 6 is required')

• Tests pass
• Appropriate changes to README are included in PR

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[antongolub]

Hey, @shevnv ,

1. Let's resolve the version value in place. Or even VERSION like on lodash
2. CLA acceptance is required

[antongolub]

@shevnv,
Do you need any help to finalize this?

[antongolub]

@shevnv,
I'll do the rest, if you dont mind.