modularize src

+ merge main.py and manage.py
+ move start logic to swingmusic/__main__.py
+ add a run.py on the project root

swingmusic/utils/__init__.py

@@ -0,0 +1,21 @@
+import locale
+from typing import Iterable, TypeVar
+
+T = TypeVar("T")
+
+# Set to user's default locale:
+locale.setlocale(locale.LC_ALL, "")
+
+# Or set to a specific locale:
+# locale.setlocale(locale.LC_ALL, 'en_US.UTF-8')
+
+
+def format_number(number: float) -> str:
+    return locale.format_string("%d", number, grouping=True)
+
+
+def flatten(list_: Iterable[list[T]]) -> list[T]:
+    """
+    Flattens a list of lists into a single list.
+    """
+    return [item for sublist in list_ for item in sublist]