Migrate language definitions to code_languages.py

[nriley]

From #1528 (comment):

In the community session today we talked further on this topic. We think we should probably have a new python file code_languages.py where we have the language definitions. This python file can then be used by the language modes and the snippets.

We want to have these language definitions typed and something like this could probably work:

string_or_list = str | list[str]
lang = tuple[str, string_or_list] | tuple[str, string_or_list, string_or_list]

languages: list[lang] = [
    ("python", ["python"]),
    ("java", "java"),
    ("javascript", ["javascript"], ["something something"]),
]