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)

[sebastian-mutz]

Thanks, @wassup05!
I'm wondering if is_abs_path may be better to use. While "path" is the first thing many will think of from context, it is entirely implied. Even in the use statement stdlib_system, only: is_abs, it's not visible (c.f. Python's os.path.isabs).

[wassup05]

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

[sebastian-mutz]

LGTM with the recent change, @wassup05. Many thanks.

[codecov]

Codecov Report

❌ Patch coverage is 58.25243% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.94%. Comparing base (1c534cb) to head (a3df735).
⚠️ Report is 41 commits behind head on master.

Files with missing lines	Patch %	Lines
test/system/test_path.f90	67.64%	22 Missing ⚠️
example/system/example_path_abs.f90	0.00%	11 Missing ⚠️
src/system/stdlib_system_path.f90	58.33%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1027      +/-   ##
==========================================
- Coverage   68.05%   67.94%   -0.11%     
==========================================
  Files         402      403       +1     
  Lines       12839    12942     +103     
  Branches     1383     1406      +23     
==========================================
+ Hits         8738     8794      +56     
- Misses       4101     4148      +47     

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