Fix NO_COLOR support

[stramel]

Fixes #38

Checks that NO_COLOR is present to disable the colors. Sorry for missing this in the spec the first time around.

@lukeed It might be nice to share this enabled logic between the 2 implementations. I'm unsure how that would affect the bench for kleur.

Also, I noticed that env.sh isn't running for tests. I had troubles with getting it to fail with the old logic or the new logic. I instead ran NO_COLOR= yarn test to see if it was disabling the colors and it appeared to work.

[codecov-commenter]

Codecov Report

Merging #39 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #39   +/-   ##
=======================================
  Coverage   98.77%   98.77%           
=======================================
  Files           2        2           
  Lines         163      163           
  Branches       31       29    -2     
=======================================
  Hits          161      161           
  Misses          1        1           
  Partials        1        1           

Impacted Files	Coverage Δ
colors.mjs	98.11% <100.00%> (ø)
index.mjs	99.09% <100.00%> (ø)

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[lukeed]

It might be nice to share this enabled logic between the 2 implementations. I'm unsure how that would affect the bench for kleur

By "2 implementations" do you mean kleur and kleur/colors? If so, definitely agree!
Won't affect bench at all. Only evaluates once as part of setup.

I have the bash script I call locally. But it's run automatically as part of VI