[BUG] Unclear semantics when using `return/break/continue` in a `finally` block

Bug description

As title.

Related:

• PEP 601 – Forbid return/break/continue breaking out of finally
• PEP 765 – Disallow return/break/continue that exit a finally block

CC @dgurchenkov.

Steps to reproduce

• If we run foo, we are greeted with Unhandled exception caught during execution: bad.
• bar prints return and exits silently.
• buz diverges and prints continue infinitely.

fn main() raises:
  foo()
  # bar()
  # buz()

fn foo() raises:
  try:
    raise "bad"
  finally:
    print("ok")

fn bar() raises:
  while True:
    try:
      raise "bad"
    finally:
      print("return")
      return

fn buz() raises:
  while True:
    try:
      raise "bad"
    finally:
      print("continue")
      continue

I'm not at all sure about what's intended behaviour, and there are many other corner cases in the linked PEPs. Maybe we should ban them from finally blocks.

System information

mojo 25.4.0.dev2025050905 (e3051145)

Start by reproducing the foo, bar, and buz examples from the issue with the listed Mojo version. Read PEP 601 and PEP 765, then compare their corner cases with the current behavior. Done requires an explicit semantic decision for return, break, and continue in finally blocks, along with behavior that consistently follows it.