Allow up to two empty lines after top-level imports (#6777)

## Summary

For imports, we enforce that there's _at least_ one empty line after an
import (assuming the next statement is _not_ an import), but allow up to
two at the module level.

Closes https://github.com/astral-sh/ruff/issues/6760.

## Test Plan

`cargo test`

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/import.py

@@ -1,3 +1,58 @@
 from a import aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa
 from a import aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa, aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa
 from a import aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa as dfgsdfgsd, aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa as sdkjflsdjlahlfd
+
+# At the top-level, force one empty line after an import, but allow up to two empty
+# lines.
+import os
+import sys
+x = 1
+
+import os
+import sys
+
+x = 1
+
+import os
+import sys
+
+
+x = 1
+
+import os
+import sys
+
+
+
+x = 1
+
+
+# In a nested scope, force one empty line after an import.
+def func():
+    import os
+    import sys
+    x = 1
+
+
+def func():
+    import os
+    import sys
+
+    x = 1
+
+
+def func():
+    import os
+    import sys
+
+
+    x = 1
+
+
+def func():
+    import os
+    import sys
+
+
+
+    x = 1