refactor: Improve non-QUIET startup time

- Change: Wake up LoadingIndicator thread when done processing sources.
  - Waiting for the thread to wake up added to the startup time.

src/termvisage/cli.py

@@ -911,6 +911,7 @@ def main() -> None:
         raise interrupt from None
 
     notify.stop_loading()
+    notify.cli_loading_interrupt.set()
     while notify.is_loading():
         pass
 

src/termvisage/notify.py

@@ -75,7 +75,7 @@ def load() -> None:
                 stream.flush()
                 if _n_loading <= 0:
                     break
-                sleep(0.25)
+                cli_loading_interrupt.wait(0.25)  # Doubles as a delay
         stream.write(" " * 3 + "\b" * 3)
         stream.flush()
         if _n_loading > -1:
@@ -180,6 +180,7 @@ def stop_loading() -> None:
 
 _loading = Event()
 _n_loading = 0
+cli_loading_interrupt = Event()
 
 # Set from `.logging.init_log()`.
 loading_indicator: Optional[Thread] = None