Add backwards compatibility scripts

[twanvl]

Currently templates have to try hard to maintain backwards compatibility. They can't remove fields or rename choices. A solution would be to allow a template to define scripts to use when a set made with an earlier version of mse or an earlier version of the template is imported.

It might look something like this:

compatibility:
    before game version: 2020-01-01
    # card fields that no longer exist or have otherwise changed
    old card field:
        name: ability1
        type: text
    # how to calculate values in this template from the old ones
    # these scripts are run once when an old set is loaded
    # can refer to both 'old' and new fields
    card field:
        name: ability_1
        script: old_card.ability1
    set field:
        name: frames
        script: if set.frames == "old choice" then "new choice" else set.frames

• Multiple compatibility blocks can be defined for different versions. All that apply are ran, from old to new version.
• Currently MSE doesn't store the game/stylesheet versions in set files. That is a first step to making this work.