Bump version to 0.12.0

[cmyr]

No description provided.

[RickyDaMa]

Because we're not at a major release yet and we're only adding new API, could we just make this a patch version? Or are you considering that the behaviour of DesignSpaceDocument::save has changed because it now writes additional fields, and thus this is breaking?

[cmyr]

Adding new fields to a struct where all fields are public is a breaking change, because you might have previously had code that either created a new struct or destructured an existing one, which will no longer compile. :(

[RickyDaMa]

Fair point, well made. I'm glad there are Cargo tools that try and automate this sort of thing, because it's nightmare-ish for mere mortals like myself 🤯