Improve definition of multi-methods

[Peter Krol]

Currently, multi-methods are collected from @tsplus fluent definitions, and overloads are ordered by collection order. In other words, they aren't ordered at all. Multi-methods should have their own distinct definition syntax that allows the definition of sort order.

Something like:

/**
 * @tsplus multi-fluent A multi 0
 */
export function multi0<A, B>(self: A, b: B) {}

/**
 * @tsplus multi-fluent A multi 1
 */
export function multi1<A, B>(self: A, b: B) {}

[Michael Arnaldi]

We could in theory consider fluent as the first implementation (implicit index 0) and require any other definition to specify an index @tsplus fluent tag name 1, similar work should be done with operators where overload selection is not yet implemented.

Additionally any duplicated tag (like two base fluent) should raise a diagnostic