Auto-detect the source code root if no paths are passed

[illright]

Currently, if you pass no paths, the CLI fails, that's not great. Make it detect the most optimal place to run and notify that this is the folder that is used as the root

[illright]

One issue with this is that this detection is quite architecture-specific, so we can't bake the FSD root detection algorithm into the Steiger core. Our choices then are:

• to simplify detection, like finding a src or app folder
• to make plugins provide their own routine for optimal start location (this kind of effort is hard to justify)