Change AuthenticationState to string literal type

[jamiehaywood]

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Purpose

• VSCode gives much nicer intellisense for string literal types over enums.
• It looks cleaner when writing the code rather than having AuthenticationState.Authenticated, you have 'Authenticated' string literal.
• No need for an import of the enum, because intellisense infers the type.

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[x] Feature
[xd] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Documentation content changes
[ ] Other... Please describe:

Other Information

This shouldn't be a breaking change, because:

AuthenticationState.Authenticated === 'Authenticated' // true