Add external AST transform support, support direct parsing of component names.

[jheer]

• BREAKING CHANGE: plugins no longer supported in article metadata, use transforms instead.
• Refactor AST transform terminology: use transforms instead of the more ambiguous plugins.
• Add transform resolve logic, enabling lookup and registration of external AST transforms.
• Add transforms monorepo package for a standard library of opt-in transforms.
• Remove knitr and pyodide transforms from compiler, move them to the new transforms package.
• Update markdown parsing to parse component names directly, remove fence and block parse context options.
• Remove code AST transform, instead annotate code languages within parser.
• Update other modules and tests in response to the above changes.