toGlobalId expects ID as a string

[intellix]

Was debugging an issue where TypeScript complains that I'm passing a number into toGlobalId and that it should be a string: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/graphql-relay/index.d.ts#L293

Decided to look upstream to this library and found that the same thing exists here:

graphql-relay-js/src/node/node.js

Lines 113 to 115 in da2801a

	export function toGlobalId(type: string, id: string): string {
	return base64([ type, id ].join(':'));
	}

The ID parameter is very likely going to be a number, especially with it being extremely common to use autoincrement integers in database rows.

Should the type be: string | number? If so, I can make the PR