[Feature Request] Set base `python` version based on `asdf set python` order

I think it would be helpful to set the base Python version (invoked when running `python`) based on the order Python versions are set. `asdf` does work as expected when using the more specific Python versions (`python3.12`, etc.), but I find it convenient to be able to type just `python` and get the version I want to use most often. I don't have `pyenv` installed anymore, but if I remember correctly, it has this behavior.

Current behavior:
```
❯ asdf set -u python 3.12.7 3.9.20

❯ python --version
Python 3.9.20
```

Desired behavior:
```
❯ asdf set -u python 3.12.7 3.9.20

❯ python --version
Python 3.12.7
```