fix: handle undefined process.env

[farnabaz]

Kleur highly uses in multiple packages like micromark and it may include in client bundle, therefore there are some cases that process.env could be undefined.

This change ensure that missing process.env does not lead to runtime exception

[lukeed]

Where / what environments does this occur? If process is defined (the line before), then process.env will also be there.

[farnabaz]

If process is defined (the line before), then process.env will also be there.

It is true in node environment, but as for client bundlers this is not valid all the time.
Some may have use cases for process but they don't provide process.env.

For example Nuxt bundler provide some info under process variable. But process.env is not defined in bundle and using kleur cause undefined error.

[Saul-Mirone]

+1, some frameworks in browser environment will remove other properties in process.

[kevinmarrec]

+1, process.env could not be available in browser, and this PR just ensure it doesn't break if it is not.

[codecov-commenter]

Codecov Report

Merging #54 (e455184) into master (598f24c) will decrease coverage by 1.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
- Coverage   98.77%   97.54%   -1.23%     
==========================================
  Files           2        2              
  Lines         163      163              
  Branches       28       30       +2     
==========================================
- Hits          161      159       -2     
- Misses          1        3       +2     
  Partials        1        1              

Impacted Files	Coverage Δ
colors.mjs	96.22% <100.00%> (-1.89%)	⬇️
index.mjs	98.18% <100.00%> (-0.91%)	⬇️

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