fix: auth property on Socket is public

[theoludwig]

Note: the socket.io.js file is the generated output of make socket.io.js, and should not be manually modified.

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

Currently the auth property on the Socket object is a private TypeScript property.
In the docs, it says that we can change the auth property. (see: https://socket.io/docs/v3/client-initialization/#auth)
But currently we can't in TypeScript.

New behaviour

Now the auth property is public, so we can update it.

[theoludwig]

I don't think, that the CI is failing because of this PR.
Is there is any reason that the auth option is private ? @darrachequesne

[darrachequesne]

Merged as c150223. Thanks!