Support SPM installation

[allenhumphreys]

Fixes #1372 #1370

To test:

You can add .package(url: "https://github.com/allenhumphreys/AztecEditor-iOS", branch: "ah/spm") to the dependencies in a sample application. If needed I can create a repo with a sample application that consumes the package.

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• [x] I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[allenhumphreys]

I just want to say thanks for the awesome library. I know I didn't ask about adding this support before opening the PR, it's ok if this doesn't align with your goal, though. I had trouble finding your project when looking to solve what this solves, and I'd love to get it listed in the swift package index if this goes through.

[erolando]

good job @allenhumphreys