core: add new `init_script_path` option: `init_script_path` option can be used in `[DEFAULTS]` to run a script at the end of backend initialisation. A new `init_pre_script` method is used to wait for backend to reach this stage (designed to be used mostly by CLI frontend), then the usual `ready_get` method is finished once the script is finished.

from typing import Optional

from twisted.words.xish import domish


class BaseError(Exception):
    namespace: str
    condition: str
    text: Optional[str]
    textLang: Optional[str]
    appCondition: Optional[domish.Element]

class StanzaError(BaseError):
    ...