Normative: Use `GetMethod`

[ExE-Boss]

This changes the algorithm to use GetMethod instead of HasProperty and Get, which has the added benefit of treating null and undefined as absent.

It also makes it so that the insert and update properties are also always consistently [[Get]]ted at the start.

Depends on:

• [x] https://github.com/tc39/proposal-upsert/pull/25 (merged)

[bmeck]

Agree on using GetMethod, but need a consensus on eager vs lazy and to write it down somewhere per

https://github.com/tc39/proposal-upsert/issues/21#issuecomment-662480082

CC: @gibson042

[dminor]

Thank you for the contribution, but I'm closing this because we're moving in a different direction with the design.