Refactor: libcrmcommon: Rename insert_prefix() to insert_spaces()

It's used to insert a prefix of (2 * depth) spaces, but typically when
we use "prefix" we mean a diff prefix (like "++") or a descriptive
prefix (like where the XML came from). This helps avoid confusion.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>