Improve project's modularization.

[oscbyspro]

The current structure cannot export text styles, as it would create cyclical dependencies. It is also ill suited for supporting additional platforms such as macOS, because DiffableTextViews becomes too crowded. A solution is to restructure the project as

Level 0: 
- DiffableTextKit

Level 1:
- NumericTextStyles
- PatternTextStyles
- DiffableTextViews_iOS
- DiffableTextViews_macOS

Level 2:
- DiffableTextViews [exports]

where the dependencies are Level 0 —> Level 1 —> Level 2 and the current DiffableTextViews target is split into two new targets: DiffableTextKit (models) and DiffableTextViews_iOS. A new DiffableTextViews then exports based on platform.

[oscbyspro]

It looks like it is not possible hide DiffableTextKit while using @_exported. Level 1 would be imported by import Level 2, while Level 0 would be importable as a separate module. It is not ideal, but it is also not a deal breaker. This realization is a product of testing .library(name: "Level 2", targets: ["Level 2"]).

[oscbyspro]

While trying to implement this I’ve been leaning towards:

Level 0: 
- DiffableTextKit

Level 1: 
- DiffableTextStyles_Numeric
- DiffableTextStyles_Pattern

Level 2:
- DiffableTextStyles [wrappers] [exports Level 1]

Level 3:
- DiffableTextViews_iOS
- DiffableTextViews_macOS

Level 4:
- DiffableTextViews [exports Level 2 and Level 3].

To avoid circular dependencies, in this case, UIKitDiffableTextStyle conformances must be made in DiffableTextViews_iOS.

[oscbyspro]

It should now be restructured similar to this in the dev_milestone_v3.0.0 branch.

[oscbyspro]

Maybe it needs to be restructured so styles have access to platform protocols (UIKitDiffableTextStyle). Thinking about it…

Level 0: 
- DiffableTextKit

Level 1:
- DiffableTextViews_iOS
- DiffableTextViews_macOS

Level 2: 
- DiffableTextStyles_Numeric
- DiffableTextStyles_Pattern
- DiffableTextStyles_Wrapper

Level 3:
- DiffableTextStyles [exports Level 2]

Level 4:
- DiffableTextViews  [exports Level 1 & 3].

[oscbyspro]

The above structure works well enough and makes it straight forward to, for example, add additional styles or a macOS target—each in a separate namespace and with explicit dependencies. It is a bit silly to have two targets that only exports, but it is what it is.