feat: path functions `is_abs` and `abs_path`

[wassup05]

User facing functions added are:

• is_abs(path): returns a logical indicating if the path is absolute.
• abs_path(path [, err]): returns the absolutized version of the path.

is_abs does pure string manipulation checking if

• on POSIX systems, path starts with /
• on Windows systems it is either an UNC path or starts with a drive letter like C:\

abs_path performs a syscall to get the current working directory and then joins it to the path, returning the joined path

Prior Art

os.path.abspath (Python) os.path.isabs (Python)

[wassup05]

I had that in mind too @sebastian-mutz, and I have changed it now.