[vim/vim] Vim9: crash when a closure assigns to a variable declared in a loop (PR #20881)

```
Problem:  Vim9: crash when a closure assigns to a variable that was
          declared in a loop (neoharju)
Solution: Encode the loop depth in the STOREOUTER instruction the same way
          as in LOADOUTER, so that the executor decodes it correctly.
```

fixes: #20877

---

### Notes

The executor decodes "outer_depth" the same way for ISN_LOADOUTER and
ISN_STOREOUTER, but generate_STOREOUTER() wrote a fixed sentinel instead of
the depth, so a loop variable was looked up in an unused out_loop[] entry
with a NULL stack.  A fixed sentinel cannot express the depth at all, so a
closure assigning to a variable of a nested loop could never have worked;
there is a test for that case as well.

The bounds check that the report suggests for the executor is not added.
The index is "-outer_depth - 1", which is the loop depth, and nesting beyond
MAX_LOOP_DEPTH is already rejected at compile time with E1306, so the index
stays within out_loop[].

OUTER_LOOP_DEPTH is removed, it has no user left.
You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/20881

-- Commit Summary --

  * Vim9: crash when a closure assigns to a variable declared in a loop

-- File Changes --

    M src/testdir/test_vim9_script.vim (30)
    M src/vim9.h (6)
    M src/vim9execute.c (7)
    M src/vim9instr.c (14)

-- Patch Links --

https://github.com/vim/vim/pull/20881.patch
https://github.com/vim/vim/pull/20881.diff

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