Add m2m100 as the new default model to support 100 languages

Added

dlt.lang.m2m100 module: Now has variables for over 100 languages, also auto-complete ready. Example: dlt.lang.m2m100.ENGLISH.
dlt.utils.available_languages, dlt.utils.available_codes: Now supports argument "m2m100"
Available languages for each model family
Script and template to generate available languages

[BREAKING] dlt.lang.TranslationModel: A new model parameter called model_family in the initialization function. Either "mbart50" or "m2m100". By default, it will be inferred based on model_or_path. Needs to be explicitly set if model_or_path is a path.
[BREAKING] Default model changed to m2m100
Docs and readme about mbart50 were reframed to take into account the new model
dlt.TranslationModel.translate: Improved docstring to be more general.
Tests pertaining to m2m100
scripts/generate_langs.py: Renamed, mechanism now changed to loading from json files
docs/index.md: Expand the "Usage" and "Advanced" sections
README.md: Add acknowledgement about m2m100, significantly trim "Advanced" section, make "Usage" more concise

dlt.TranslationModel.available_codes() was returning the languages instead of the codes. It will now correctly return the code.

Output type hints for TranslationModel.get_transformers_model and TranslationModel.get_tokenizer
[BREAKING] dlt.TranslationModel.bart_model and dlt.TranslationModel.tokenizer are no longer available to be used directly. Please use dlt.TranslationModel.get_transformers_model and dlt.TranslationModel.get_tokenizer instead.