Add source generation for commands

[fernandreu]

Instead of having to type the boilerplate of;

private RelayCommand? _insertIconsCommand;
public RelayCommand InsertIconsCommand => _insertIconsCommand ??= new RelayCommand(ExecuteInsertIconsCommand);

use an attribute for the method that does this for you:

[GenerateCommand]
private void ExecuteInsertIconsCommand()
{
    ...
}

This should be 100% feasible with source generators. These should also:

• Check return type to determine whether to use a RelayCommand or an AsyncCommand
• Check the method parameter list and restrict its usage to methods with either 0 or 1 parameters
• Adopt some naming convention (e.g. remove Execute / Async from the method name and append Command), overrideable with an attribute parameter
• Deal with generics well, e.g. RelayCommand<SomeType<SomeNestedType>>>
• Pick the docstring from the method, or provide an attribute property to specify your own
• Report diagnostics for any issue that might occur so that anything going on doesn't become too obscure
• They will ignore the CanExecute callbacks for now, since this tool does not make use of them. However, at some point they could also be specified in another attribute property, via nameof(TheCanExecuteMethod)