buildozer: add new_if_absent command

[amartani]

This change adds a new command 'new_if_absent' to buildozer.

The 'new_if_absent' command behaves the same as the 'new' command, but it does not fail if a rule with the same name already exists. Instead, it performs a no-op.

This is useful for scripts that want to ensure a rule exists without having to check for its existence first.