Append/prepend: search for full line
Append/prepend: search for full line

When `append_if_not_found` or `prepend_if_not_found`
are true, assume the `content` to search for should
match a full line in the file. The rationale is that
if the content is not found, then it will be added as
a full line. Hence, it should be safe to assume that
it should exist as a full line in the file.

Fixes #23294

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt