feat: added the warnRuleAsWarning option, allow to emit warnings on…

[alexander-akait]

… @warn rule

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

fixes #588

Breaking Changes

No

Additional Info

It will be true by default in the next major release

[codecov]

Codecov Report

Merging #992 (33241a6) into master (bb7cef9) will decrease coverage by 0.20%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #992      +/-   ##
==========================================
- Coverage   96.12%   95.91%   -0.21%     
==========================================
  Files           4        5       +1     
  Lines         284      294      +10     
  Branches       94       97       +3     
==========================================
+ Hits          273      282       +9     
- Misses         10       11       +1     
  Partials        1        1              

Impacted Files	Coverage Δ
src/SassError.js	100.00% <ø> (ø)
src/SassWarning.js	83.33% <83.33%> (ø)
src/utils.js	95.52% <100.00%> (+0.07%)	⬆️

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