Fix missing newline in error messages for some sources.

[mfelsche]

Pull request

Description

If the source line with the error didnt end with a newline (e.g. last line), we missed to add one, so the callout starts on its own line. This is fixed here (with some horrible workaround for term highlighting).

Related

Checklist

• The RFC, if required, has been submitted and approved
• Any user-facing impact of the changes is reflected in docs.tremor.rs
• The code is tested
• Use of unsafe code is reasoned about in a comment
• Update CHANGELOG.md appropriately, recording any changes, bug fixes or other observable changes in behavior

[coveralls]

Coverage increased (+0.006%) to 80.783% when pulling bde49ae on callout-newline into 4538891 on main.

[Licenser]

Aside from some tests and a CHANGELOG entry 👍

[Licenser]

Perfect 👍