No way to install Julia signal handler when other dependency uses juliacall

**Affects:** JuliaCall

**Describe the bug**

Related to https://github.com/JuliaPy/PythonCall.jl/issues/219. There's currently a scenario with no possible workaround other than forcing a downstream user to set up environment variables to avoid a segfault.

Say that I import a package which uses serial Julia code. That package runs

```python
from juliacall import Main as jl
```

to start Julia.

Then, I import a different package. That package calls multithreaded Julia code, and thus to avoid a segfault (https://juliapy.github.io/PythonCall.jl/stable/faq/#Is-PythonCall/JuliaCall-thread-safe?) instead performs the import instead as:

```python
import os
os.environ["PYTHON_JULIACALL_HANDLE_SIGNALS"] = "yes"

from juliacall import Main as jl
```

However, at this point, the Julia runtime has already started. So now when I run a multithreaded portion of code, I will get a segfault, even though I implemented special workarounds in the multithreaded package.

How could I avoid this scenario? Indeed the user could always set up the environment variables themselves. But managing to figure this out from a random `bus error` will only be ~5% of users.

Start by reading JuliaCall's initialization path around `from juliacall import Main as jl` and the `PYTHON_JULIACALL_HANDLE_SIGNALS` environment variable, then review the linked PythonCall.jl issue and thread-safety FAQ. Done means a package can safely arrange signal handling when another dependency has already started the Julia runtime, without requiring downstream users to discover environment-variable workarounds.

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