This package is available on OpenUPM: https://openupm.com/packages/com.brunomikoski.editorhistorypanel

To add it the package to your project:

• open Edit/Project Settings/Package Manager
• add a new Scoped Registry:

  Name: OpenUPM
  URL:  https://package.openupm.com/
  Scope(s): com.brunomikoski
  

• click Save
• open Package Manager
• click +
• select Add from Git URL
• paste com.brunomikoski.editorhistorypanel
• click Add

You can also add it directly from GitHub on Unity 2019.4+. Note that you won't be able to receive updates through Package Manager this way, you'll have to update manually.

• open Package Manager
• click +
• select Add from Git URL
• paste https://github.com/brunomikoski/Animation-Sequencer.git
• click Add