Source code artifacts: support multiple languages

[hlibbabii]

Currently, when the Method artifact is used, it's assumed that it is a java method, e.g. _get_max_depth method is java-specific:

bohr/bohr/framework/artifacts/method.py

Line 27 in c1b340b

def _get_max_depth(self) -> int:

We would want to reuse the common interface of the Method artifact (and other source code artifacts) for multiple languages, possibly implementing language-specific logic in sub-classes