[JS] Support nullish coalescing assignment operator (??=)

According to https://docs.aws.amazon.com/appsync/latest/devguide/supported-features.html, these operators are supported:

> - standard math operators (+, -, /, %, *, etc.)
> - nullish coalescing operator (??)

As it turns out, the assignment version of these operators are only supported for the standard math operators. I.e.

- **Supported:** +=, -=, /=, %=, *=, etc.
- **Not Supported:** ??=

`??=` is very useful, so it would be great if we could support that as well

Start by reviewing the AWS AppSync supported-features documentation linked in the issue and the JavaScript runtime's existing support for nullish coalescing and compound assignment operators. Confirm the desired behavior for ??= and identify the runtime or compatibility test entry point; done means ??= is accepted and behaves correctly in AppSync JavaScript resolvers.

Written by the indexing model from the issue text.