Ensure writing changelogs will succeed before writing files about changesets

[Noviny]

I think this wants some minor/major changes to how bumping works. Currently file writing gets handed over to whatever function edits the file. Instead, all file edit functions return the new contents and the file path in an object, but don't write.

Finally, an end process does all file writing.

Bonus, we can add a check to make sure in any bump, we only have one instance of a file to write.

New pattern in bump effectively looks like:

const bump = (config) => {
  /* some setup stuff */

  let { errors: changelogErrors, files: changelogFiles } = await updateChangelogs(config)
  let { errors: pkgJsonErrors, files: pkgJsonFiles } = await updateVersions(config)

  if (changelogErrors || pkgJsonErrors ) / * handle this */

  await writeFiles([...changelogFiles, ...pkgJsonFiles])
}