fixes #19345

fixes #15657

honor order when merging contributed menu items with legacy items, legacy items have order zero, #9153

make menu item order explicit with 'order' property (default to 0)
make group order be the natural string order with exceptions for 'navigation' (always first) and falsy groups (always last)

see https://basarat.gitbooks.io/typescript/content/docs/tips/nominalTyping.html and https://github.com/Microsoft/TypeScript/blob/7b48a182c05ea4dea81bab73ecbbe9e013a79e99/src/compiler/types.ts#L693-L698