Add Characters to LibreLingo python scripts

[Skivling]

This is the preliminary work for #1143, implementing them in Python. Later, once @kantord is up to it in the web app rewrite they can be implemented there.

I have made some assumptions for what best to name the titles in Yaml and functions, which I am fine changing if there is a better option.

I have also added a property in Characters for their International Phonetic Alphabet (IPA) pronounciations, as I believe this would be useful to have below or above the characters for some learners, especially those who are very into linguistics and/or learning a conlang in a LibreLingo course.
The IPA value is not required in the schema so not including it won't cause any errors.

I hope unicode and such doesn't cause any bugs or errors such as a new character not displaying properly. 🤞

Currently Character()s in a skill look like this:

New Characters:
   
   - Character: ā
     Transliteration: aa
     IPA: /aː/
   
   - Character: Г
     Transliteration: H
     IPA: /ɦ/

And here they are in Python:

    # A long sound macron on an 'a' in Te Reo Māori
    >>> Character("ā", "aa", "/aː/")
    Character(character='ā', transliteration='aa', ipa_pronounciation='/aː/')

    # The Г character in Ukranian (which uses the Cyrillic script).
    >>> Character("Г", "H", "/ɦ/")
    Character(character='Г', transliteration='H', ipa_pronounciation='/ɦ/')

I have yet to finish the in-code documentation of the functions, then adding all this to the proper docs, hence this PR is a draft. I also haven't tested anything yet.

[Skivling]

I have made the IPA field a list because it's quite common for a character to represent a few distinct but similar sounds. Same as the transliteration field.

Should the transliteration field be renamed as Transliterations (plural) to indicate this?