vim-patch:9.1.0999: Vim9: leaking finished exception (#31939)

Problem:  leaking finished exception
          (after v9.1.0984)
Solution: use finish_exception to clean up caught exceptions
          (Yee Cheng Chin)

In Vimscript, v:exception/throwpoint/stacktrace are supposed to reflect
the currently caught exception, and be popped after the exception is
finished (via endtry, finally, or a thrown exception inside catch).
Vim9script does not handle this properly, and leaks them instead. This
is clearly visible when launching GVim with menu enabled.  A caught
exception inside the s:BMShow() in menu.vim would show up when querying
`v:stacktrace` even though the exception was already caught and handled.

To fix this, just use the same functionality as Vimscript by calling
`finish_exception` to properly restore the states. Note that this
assumes `current_exception` is always the same as `caught_stack` which
believe should be the case.

Added tests for this. Also fix up test_stacktrace to properly test the
stack restore behavior where we have nested exceptions in catch blocks
and to also test the vim9script functionality properly.

- Also, remove its dependency on explicitly checking a line number in
  runtest.vim which is a very fragile way to write tests as any minor
  change in runtest.vim (shared among all tests) would require changing
  test_stacktrace.vim. We don't actually need such granularity in the
  test.

closes: vim/vim#16413

https://github.com/vim/vim/commit/2051af1642843426714efc2572c3e270fe0948be

Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

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