Decorators on getter/setter ignores implementation, allows duplicates

[wjhsf]

class Component extends LightningElement {
  @wire(getBook, {id: 1}) get book1 () {
    console.log('getter')
  }
  @wire(getBook, {id: 2}) set book2(v) {
    console.log('setter')
  }
}

Currently, the compiler treats getters and setters identically to props; the implementations present on a component are dropped. (In the example above, neither 'getter' nor 'setter' are ever logged to console.) This behavior is not obvious, but using @wire on a getter/setter is also kind of weird. (A getter cannot receive data, so decorating it with a data provider doesn't really make sense.) To make the behavior more consistent with expectations, we should either disallow @wire on a getter/setter, or we should preserve their implementations. In either case, it would be a breaking change.

[nolanlawson]

I am going to cram all the decorator-related issues with getters/setters into this issue. In short:

• We allow duplicate props/methods/getters/setters when we should throw an error at compile time instead
• Getters/setters are not actually supported and just become props at runtime

[nolanlawson]

As a first step, we should probably just warn on duplicates. This is low-to-zero-risk.