Add `jlx`

This could be similar to `npx`, i.e. to run Julia applications. I haven't thought much about it, but I think my main scenario would be something like:
```
jlx Foo
```
would automatically install a package called `Foo` from the general registry, and then run some well defined entry point (say `Foo.main`).

No file or test is identified in the issue. Start by reviewing how juliaup defines command entry points, then clarify the proposed `jlx Foo` behavior, package installation from the general registry, and the `Foo.main` convention before implementation.

Written by the indexing model from the issue text.