[New feature] Add an abstract importer

[jecisc]

Most FAST project will have an importer class and they will have most of their code in common.

I'll propose to integrate an abstract class to factorize this code and reduce the implementation effort for new FAST importers

[alesshosry]

True, I will handle this to remove the duplication; I think I will add the importer in pharo-tree-sitter and make each FASTLanguageParser define the TsLanguage and Language name, and use the importer added newly in Pharo tree sitter since it's common for all languages.

[jecisc]

I have a branch where I fixed this, I'll propose a PR when the customizable importer will be more stable :)
I opened issues to keep track of things