Add support for filter predicate in once() (#122)

[faizanu94]

Add support for filter predicate in once()

This change extends once(eventNames, predicate) so you can pass an optional filter function. When provided, once will:

• ignore any events for which predicate(data) returns false
• unsubscribe and resolve on the first event where predicate(data) returns true

If no predicate is passed, once behaves exactly as before. The predicate is validated up-front and will throw a TypeError if it’s not a function. Four new tests cover:

• resolving only on the first matching event
• throwing on invalid predicate types
• using multiple event names
• unsubscribing via promise.off() before a match

Closes #122

[sindresorhus]

You need to update the readme and index.d.ts

[faizanu94]

@sindresorhus I’ve updated the readme and index.d.ts accordingly