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

[saltman424]

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