Support Symbol as eventName

[sindresorhus]

See #12 (comment) for use-case.

I think #13 should be implemented first though, so we can make sure it's not hard to define in a TS type definition.

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• #43 Allow symbol event names

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[novemberborn]

We'll be able to support this in the Emittery class itself, but TypeScript cannot yet express symbol types sufficiently for use in the Emittery.Typed class. That's OK though, we can definitely tackle this now.

[sindresorhus]

TypeScript 2.7 is now out with support for Symbol: https://blogs.msdn.microsoft.com/typescript/2018/01/31/announcing-typescript-2-7/#unique-symbol-types-and-const-named-properties

[dinoboff]

AFAIK, the support is still limited: they cannot be use with keyOf or as index signature parameter. It can be used in overloaded method signatures however.

[sindresorhus]

Relevant TypeScript issues:

• Consider adding symbolof type operator, like keyof but for unique symbol properties microsoft/TypeScript#20721
• keyof any incorrectly maps (only) to type string microsoft/TypeScript#21983

[sindresorhus]

The above issues has been fixed.

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