Control flow: return and raise (#17121)

We add support for `return` and `raise` statements in the control flow
graph: we simply add an edge to the terminal block, push the statements
to the current block, and proceed.

This implementation will have to be modified somewhat once we add
support for `try` statements - then we will need to check whether to
_defer_ the jump. But for now this will do!

Also in this PR: We fix the `unreachable` diagnostic range so that it
lumps together consecutive unreachable blocks.

crates/ruff_python_semantic/resources/test/fixtures/cfg/jumps.py

@@ -0,0 +1,18 @@
+def only_return():
+    return
+
+def after_return():
+    return 1
+    print("unreachable")
+    print("and this")
+
+def after_raise():
+    raise ValueError
+    print("unreachable")
+    print("and this")
+
+def multiple_returns():
+    return 1
+    print("unreachable")
+    return 2
+    print("and this")