Add experimental TypeScript support

[GarboMuffin]

• Teach the build server to compile .ts to .js as needed.
• .ts will have hard requirement to pass type checking; .js remains warn-only
• A few extensions were converted to .ts to see how well it works in practice

remaining:

• linting integration
• docs? or keep it "secret" for now
• better template that can do more type magic for us to fix these kinds of issues at compile time:
  • cloudlink: fix referencing nonexistent parameter #2579
  • Various fixes related to extensions referring to non-existent arguments #2585
  • Xeltalliv/clippingblending: Fix sprites which had pen down before extension was imported #2578