Provide a more extensive default bootstrap

[freakboy3742]

What is the problem or limitation you are having?

If a user selects "No GUI framework", no bootstrap content is selected. As a result, the generated app configuration is really bare bones. This puts the onus on the developer to discover all the important configuration keys that they might want to use.

Describe the solution you'd like

A new "Empty" bootstrap that contains all the same configuration keys as the Toga backend, but with no values specified. This would be used in the New and Convert wizards.

For example, the content for each operating system section (e.g., macOS, Linux, ...) would be something like:

# Delete the next line to support macOS
supported = false
requires = [
    # Python requirements for macOS
]

The Debian configuration section might be:

system_requires = [
    # System packages needed to run the app on Debian
]

system_runtime_requires = [
    # System packages needed to run the app on Debian
]

There would be no app content; this is purely to ensure that there are empty stub configurations for each platform that the user can "fill in", rather than needing to read the docs (and potentially hit typos etc).

Describe alternatives you've considered

Do nothing. The status quo works, and the empty template case is an edge case.

Additional context

Bootstrap definitions are in src/briefcase/bootstraps; they're registered in pyproject.toml, and installed in NewCommand._gui_bootstrap_choices() and ConvertCommand.build_gui_context().