[Windows] Support modifying an existing installation

[tristanlabelle]

As the number of optional features in the installer grows, it makes it more likely that a user would want to modify their installation to add or remove features. Currently, the installer supports the “Repair” action but not “Modify”. We would want to support that flow, during which the user is presented with the options UI again, where they can see the currently installed features and select which features to add or remove.