More plugins: splashscreen and importmap (#938)

This PR move codes from main.ts into two new plugins:

- splashscreen (formerly known as py-loader)
- importmap

The old setting config.autoclose_loader is still supported but deprecated; the new setting is config.splashscreen.autoclose.

Moreover, it does a small refactoring around UserError: now UserErrors are correctly caught even if they are raised from within afterRuntimeLoad.

examples/simple_clock.html

@@ -21,8 +21,6 @@
       files = ["./utils.py"]
     </py-config>
     <py-script output="outputDiv">
-# demonstrates how use the global PyScript pyscript_loader
-# to send operation log messages to it
 import utils
 display(utils.now())
     </py-script>
@@ -43,8 +41,6 @@ async def foo():
     else:
       out3.clear()
 
-# close the global PyScript pyscript_loader
-pyscript_loader.close()
 pyscript.run_until_complete(foo())
     </py-script>
   </body>