Adopt the use of PEP 621 entry points

### What is the problem or limitation you are having?

Briefcase uses an assumption that an app will always be started using the app name as a module (i.e., the analog of `python -m `).

However, PEP 621 provides for entry point definitions - the ability to define what public entry points should be defined when a package is installed. This also allows for both GUI scripts and console scripts.

Briefcase should

### Describe the solution you'd like

It should be possible to use a PEP 621 entry point definition for an app. The current Briefcase default behavior should be the equivalent of an explicit:
```
[project.gui-scripts]
my-app = "my_app"
```

(or an even more explicit)
```
[project.gui-scripts]
my-app = "my_app/__main__:main"
```

"Invoke the app name as a module" should be retained as a default behavior. However, if the user defines a PEP 621 entry point matching the app name, that would override the default behavior.

This would also supersede the `console_app` definition. If the app name is defined in `[project.scripts]`, it would be assumed to be a console script. Defining the same app in both `gui-scripts` and `scripts` would be considered a configuration error. Defining `console_app` at all if an entry point is explicitly provided would also be a configuration error.

An app should also be able to define a `scripts` or `gui-scripts` key, defining the list of entry points that this app will expose. The default behaviour would effectively be the equivalent of:

```
[project.gui-scripts]
my-app = "my_app"

[tool.briefcase.app.my-app]
gui-scripts = ["my-app"]
```

As a first pass, this list should be limited to a single entry (and may always be limited to a single entry on some platforms - macOS, iOS and Android only allow a single GUI entry point in an app), with longer lists raising an error. However, in time, we may be able to accommodate multiple entry points in a single "app".

### Describe alternatives you've considered

Status quo.

Start by tracing Briefcase's app configuration handling and how it currently invokes an app as a module. Map the PEP 621 [project.scripts] and [project.gui-scripts] definitions to the app's configured entry-point list, including validation for conflicting scripts, console_app, and lists longer than one. Done means explicit entry points override the default while invalid combinations raise configuration errors.

Written by the indexing model from the issue text.