move functionality into package extensions

[cjdoris]

Can probably make these into weak dependencies:

• Serialization
• Dates
• Tables
• Markdown??
• Pkg??
• REPL??

Also:

• make PyCall a weak dependency? Done!
• remove Requires? Done!
• require Julia 1.9? or just have older versions be less compatible? or use PackageExtensionCompat? Done!