Add support for the css-variables theme to ansi rendering

[blake-mealey]

I realized while integrating the recent ANSI support into Nextra that Nextra uses the css-variables functionality which was not working with ANSI colors. I've updated the default color replacements and css-variables theme to support ANSI colors and updated the ANSI tokenizer to use a separate -dim variable for dimmed colors since we can no longer add opacity ourselves.

• Add a test if possible
• Format all commit messages with Conventional Commits

• This PR fixes #

• I have read docs for adding a theme.

• I have read docs for adding a language.
• I have searched around and this is the most up-to-date, actively maintained version of the language grammar.
• I have added a sample file that includes a variety of language syntaxes and succinctly captures the idiosyncrasy of a language. See docs for requirement.

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[blake-mealey]

@octref would you please take a look at this PR?

[octref]

Looking good. Thanks for the contribution and sorry for my delay.