Sanitize File\exists? + add missing stdlib function

[drkameleon]

Description

Right now, we have exists?, which works in the following way:

• exists? "some/path" - will return true if the path exists and points to a file, but false if the path exists but points to a directory or anything else (or: ofc, if the aforementioned path doesn't exist altogether)
• exists?.directory "some/path" - will return true if the path exists and point to a directory, but false if the path exists but points to a file or anything else (or: ofc, if the aforementioned path doesn't exist altogether)

That sounds a bit confusing. (See here: https://discord.com/channels/765519132186640445/829324913097048065/1208304920957157386)

Basically, if we want to check if a given path is a directory, we'd have to do exists?.directory - which is correct, although it's not ultra-intuitive.

What this PR is going to do is:

• make sure that exists? returns true if the path exists (be it a file, a directory or whatever) and false otherwise
• create at least a file? & directory? predicate/function to check if a given path is a file or directory, respectively. (We could perhaps add another symlink? function, since it's a thing and Nim allows us to do this quite easily 😉 )

Type of change

• Enhancement (implementation update, or general performance enhancements)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (documentation-related additions)

[drkameleon]

Ready to merge! 🚀