Add functionality to modify an existing manifest

[ryfu-msft]

As of right now, we only have the new command to generate new manifests and the update command to update installer urls, version, and hash. There is a need for a modify command that pulls down an existing manifest and allows a user to make changes to any field they want to edit.

This would address issues like #75 by allowing users to replace installers and their related fields. We currently don't support replacing existing installers with new ones in order to preserve updating multiple installers and avoid accidentally replacing the wrong installer.

The modify command flow would look something like this:

wingetcreate modify <Package Identifier> -v <Version>

• Pulls down the manifests for the specified package and the specified version (if no version is specified, latest manifest is pulled down)
• Prompts would show for each field, with some form of selection logic to allow users to edit by manifest, installer node, field, etc.
• Saves newly modified manifests locally and also prompts the user if they would like to submit to the Winget-Pkgs repository.