Eleventy Syntax Highlighter v4.2.0

• Updates from prismjs 1.26.0 to 1.29.0
• Adds syntax-highlight.webc WebC syntax highlighter component, in-use on https://github.com/11ty/demo-webc-image-compare/blob/358777337366dd2426c064c7bf2b200f10df98a5/index.webc (example usage)
• Adds GitHub actions to run the test suite.

Full Changelog: v4.1.0...v4.2.0