[vim/vim] Can't use 'is' in a oneline vim9 lambda (Issue #20967)

mao-yining created an issue (vim/vim#20967)

### Steps to reproduce

lagacy `{-> '2' is# '2'}()` runs well
vim9 `() => '2' ==# '2'` runs well
vim9 `() => '2' is# '2'` thrown `E15`

### Expected behaviour

no `E15`

### Version of Vim

9.2.843

### Environment

Fedora 44, gVim

### Logs and stack traces

```shell

```

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