Fix detection of dumb terminals on Windows

[MartinKolarik]

The default Windows terminal doesn't set env.TERM so we cannot require the variable to be set like on other platforms but if it is set, we should use it because there are many third-party terminals, particularly those embedded in various GUI apps, that don't support colors and correctly set this variable. Fixes #90.

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[codecov-commenter]

Codecov Report

Merging #91 (3a63878) into main (840f50e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #91   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          145       146    +1     
=========================================
+ Hits           145       146    +1     

Impacted Files	Coverage Δ
index.js	100.00% <100.00%> (ø)

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[kibertoad]

Thank you!

[jorgebucaran]

Thank you, @MartinKolarik, @kibertoad! 🙌