[Feature Request] Return changelog diff with code usage

[crutch12]

standardVersion() should return Promise<string> with calculated diff so we could use its result programmatically.

Now it returns undefined (there is no return)

Usage example:

const standardVersion = require('standard-version')
const gitLog = await standardVersion({
  noVerify: true,
  silent: true,
  dryRun: true,
})
  .then((result) => {
    console.log(result); // it's undefined now and there is diff changelog message in console output
    return result; 
  })

Expected output (returned value)

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## [1.0.0-next-release](2022-05-12)

### Features

* [OFKR-2125]  did smth
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