Support for typechecking in usage of fqsl

[tmi]

A few things of interest:

• addition of py.typed. This tells mypy that the package is aware of types and those should be respected
• addition of MANIFEST.in and include_package_files in setup.cfg -- this is so that the py.typed actually gets used in build (but in a way that the .build-venv is not included)
• move of the ./fsql to ./src/fsql -- this will make it easier once we create some sub-module inside fsql (we would have to include it by hand in setup.cfg otherwise). This triggered small updates in setup.cfg and isort.cfg

[vojtechtuma-imi]

huh this one got forgotten it seems... @ampx-mg do you want to have a look and possibly approve?

[ampx-mg]

lgtm!