This pull request introduces enhancements to the Lexer class to support CSS variable (var()) token replacement and matching. The most important changes include adding a new function for variable token replacement, updating the matchSyntax function to handle variable tokens, and modifying the matchProperty and matchDeclaration methods to accept variable values.

The basic idea is to let consumers pass in strings or nodes that would be used in place of the variable reference. I opted to create an options object rather than variables object to leave space for further expansion.

There are a couple of failing tests that I couldn't figure out, but at this point, just looking for feedback on if this approach would be acceptable.

Fixes #317

Enhancements to CSS variable handling:

• lib/lexer/Lexer.js: Added the replaceVarTokens function to replace var() tokens with their corresponding values from a provided variables object.
• lib/lexer/Lexer.js: Updated the matchSyntax function to utilize replaceVarTokens for handling var() tokens and returning an error if the variable value is not defined. [1] [2]
• lib/lexer/Lexer.js: Modified the matchProperty and matchDeclaration methods to accept an additional vars parameter for variable values. [1] [2]

Testing for new functionality:

• lib/__tests/lexer-match-property.js: Added tests to verify the behavior of matchProperty when handling var() tokens, both with and without provided variable values.