Tokenise CSS variables

[darrenburns]

Implements tokenisation of SASS-like variables.

Variables can be declared with values at the root scope, and you can refer to these variables inside any rule or variable declaration.

[codecov-commenter]

Codecov Report

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@@          Coverage Diff           @@
##             css     #250   +/-   ##
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  Coverage       ?   37.74%           
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  Files          ?       84           
  Lines          ?     6449           
  Branches       ?        0           
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  Hits           ?     2434           
  Misses         ?     4015           
  Partials       ?        0           

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unittests	37.74% <0.00%> (?)

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[darrenburns]

@willmcgugan have modified the tokenisation as agreed

[willmcgugan]

Excellent. And well tested!