Add __main__ to re-enable demo via python -m termcolor

[hugovk]

Running python -m termcolor with termcolor 1.1.0 gave a demo:

That broke after moving to src layout in 4538442 in termcolor 2.0.0:

$ python -m termcolor
/Library/Frameworks/Python.framework/Versions/3.11/bin/python3: No module named termcolor.__main__; 'termcolor' is a package and cannot be directly executed

So let's move that code into termcolor.__main__ to re-enable the python -m termcolor demo.

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (315e658) compared to base (9f42863).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #27   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          106       106           
=========================================
  Hits           106       106           

Flag	Coverage Δ
macos-latest	100.00% <ø> (ø)
ubuntu-latest	100.00% <ø> (ø)
windows-latest	100.00% <ø> (ø)

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Impacted Files	Coverage Δ
src/termcolor/termcolor.py	100.00% <ø> (ø)

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