Use `Scalar.source` when serialising numerical values

[eemeli]

By default, numerical values (!!int and !!float) are represented using the native JS Number, which has limits to the precision and absolute value of what it can represent before overflowing:

import { parseDocument } from 'yaml'

const doc = parseDocument('gitsha: 61e9540')
doc.get('gitsha', true)

Scalar {
  value: Infinity,
  range: [ 8, 15, 15 ],
  source: '61e9540',
  type: 'PLAIN',
  format: 'EXP'
}

Note, however, that we already retain the string source of the value. When serialising the above, we should make use of it, so that we don't end up with:

gitsha: .inf