Add stylesheet for light/dark mode code syntax highlighting

[benhorsburgh]

Description

Addition of a new highlightjs.css stylesheet that provides Vizro-styled syntax highlighting in both light and dark mode.

Fixes #1525

Screenshot

Light mode

Dark mode

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[Copilot]

Pull request overview

This PR adds a new CSS stylesheet for code syntax highlighting using highlight.js, providing Vizro-styled colors that work in both light and dark themes. Additionally, the PR adds a new contributor to the authors list.

Key changes:

• New highlightjs.css file implementing syntax highlighting color schemes using CSS variables
• Addition of "Ben Horsburgh" to the contributors list in authors.md

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 10 comments.

File	Description
vizro-core/src/vizro/static/css/highlightjs.css	New CSS file defining color scheme for highlight.js syntax highlighting with Vizro theme variables
vizro-core/docs/pages/explanation/authors.md	Added new contributor Ben Horsburgh to the contributors list

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