fix: proposed token important fix

[shellscape]

Which issue # if any, does this resolve?
#118

Please check one:

• New tests created for this change
• Tests updated for this change

This PR:

• Adds new API
• Extends existing API, backwards-compatible
• Introduces a breaking change
• Fixes a bug

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From #124

This should be a better approach to it than #119, the logic is mostly from core postcss. I'm hoping to find a way to consolidate it to one solution across both libraries.

This PR supersedes #124. It uses a different approach which alters the tokenizer when important identifiers are split between multiple tokens.

[codecov-io]

Codecov Report

Merging #126 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   96.09%   96.36%   +0.26%     
==========================================
  Files           8        8              
  Lines         205      220      +15     
==========================================
+ Hits          197      212      +15     
  Misses          8        8

Impacted Files	Coverage Δ
lib/LessParser.js	99.07% <100%> (+0.14%)	⬆️

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[shellscape]

@jwilsson updated those two tests