Updates to Database Type Definitions

[hiranya911]

• Expose ThenableReference type from the database type definitions.
• Currently then() and catch() methods are exposed from the Reference type, which doesn't seem to be correct. Those belong in ThenableReference.
• Add EventType type:

type EventType = 'value' | 'child_added' | 'child_changed' | 'child_moved' | 'child_removed';

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[jspri]

then and catch still exist on all references https://github.com/firebase/firebase-js-sdk/blob/master/packages/database/src/api/Reference.ts#L45

but the exposed types were fixed in #334.

type EventType = 'value' | 'child_added' | 'child_changed' | 'child_moved' | 'child_removed';

Is the only point that still needs to be addressed.