docs(changelog): add changelog file

[SimJoSt]

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Add a changelog file based on https://keepachangelog.com/ This helps document changes more clearly, centralized and without relying on the add-on and extension directories of the browsers to keep and list them.

[SimJoSt]

@wildskyf like we discussed, I want to contribute a changelog to the project. To retroactively add all old versions, I went through the commit log and tagged all commits appropriately: https://github.com/SimJoSt/TextareaCache/tags If you want to import them to this upstream repo, you can do so. I invited you as a collaborator to my fork. If you want me to do it for you, I can help out, too. With the tags imported and the changelog, it would also be possible to easily get the GitHub releases up-to-date as well.