Support multi-language files

[rien]

Using tree-sitter it is possible to parse multi-language documents such as ERB, HTML, Vue, React, ... However, it needs non-trivial interventions on our side, see: https://tree-sitter.github.io/tree-sitter/using-parsers#multi-language-documents

The python-notebook prototype (https://github.com/dodona-edu/dolos/pull/486) could be used as inspiration for this change.