Enable `-Wall -Wextra -Wshadow -Wconversion -Wpedantic`

[SuperFola]

I tend to forget about Werror because of external libraries which aren't compliant. We used to have utf8_decoder (but now it's gone) that didn't comply ; IIRC replxx isn't either.

Some tests will have to be conducted through the CI with those flags directly enabled inside the CMakeLists (so that they are useful locally too).