Deprecate unbound method usage

[jonkoops]

Public methods on the Keycloak class were converted to arrow function class fields in #204 to support destructuring (e.g. const { login } = keycloak). While this works, arrow fields have downsides: no prototype sharing, no reference equality across instances, and broken subclassing.

This issue tracks adding deprecation warnings when methods are called with a lost this context, so that in the next major (#234) we can remove the rebinding and use proper prototype methods.

Related:

• Release 26.2.1 contains breaking changes #202 — Original issue that prompted the arrow field change
• Bind public methods to class instance #204 — PR that introduced the arrow fields
• Remove re-binding of public methods #234 — Next major: remove re-binding of public methods