Support general I/O for ProgressBar output

[ytdHuang]

This PR allows users to output ProgressBar into general I/O, useful for writing logs or even into files.

Summary of this PR:

• output next!() into general IO (default to stdout)
• export next!()
• Improve runtests coverage for ProgressBar
• change variable name test into core_test in file test/runtest.jl

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.87%. Comparing base (7cd969c) to head (48e674c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   89.03%   89.87%   +0.84%     
==========================================
  Files          18       18              
  Lines        1660     1660              
==========================================
+ Hits         1478     1492      +14     
+ Misses        182      168      -14     

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