Align How Enums are Defined

[takameyer]

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What, How & Why?

This addresses a breaking change we want to make where typed "string" unions are replaced with respective enums.

This closes #1955

☑️ ToDos

• [ ] 📝 Changelog entry
• [ ] 📝 Compatibility label is updated or copied from previous entry
• [ ] 🚦 Tests
• [ ] 🔀 Executed flexible sync tests locally if modifying flexible sync
• [ ] 📦 Updated internal package version in consuming package.jsons (if updating internal packages)
• [ ] 📱 Check the React Native/other sample apps work if necessary
• [ ] 📝 Public documentation PR created or is not necessary
• [ ] 💥 Breaking label has been applied or is not necessary

If this PR adds or changes public API's:

• [ ] typescript definitions file is updated
• [ ] jsdoc files updated
• [ ] Chrome debug API is updated if API is available on React Native

[kneth]

@tomduncalf @cmelchior Please be aware that the changes to Realm.Auth.EmailPasswordAuth are cover by a private Jira ticket.

[kraenhansen]

It looks like we forgot the Realm#addListener which takes a string for event name, which should actually take an enum 🤔

https://github.com/realm/realm-js/blob/master/types/index.d.ts#L1265-L1285