Validate that current release is present in changelog

[Gudahtt]

The validate command does not validate that the current release exists in the changelog (unless the --rc flag is used). Some of our publishing workflows depend upon the release existing, so we need this tool to help us ensure it's present.

The validate command already checks this correctly in --rc mode.

Acceptance criteria:

• When run without the --rc flag:
  • When the release is 0.0.0:
    • auto-changelog validate throws an error if the release is present in the changelog (0.0.0 should indicate that the package has not yet been released)
  • When the release is anything other than 0.0.0"
    • auto-changelog validate throws an error when the current release is missing
• We have unit tests for both cases