Add environment variables support for JS resolvers

When a script needs some information about other resources, the primary way is to use environment variables. For example, I have this Terraform script:

```
resource "aws_appsync_function" "Mutation_pairUsers_1" {
# ...
code = <

Start with the Terraform aws_appsync_function example and the JavaScript resolver entry point shown in the issue. Determine how values such as the DynamoDB table name should become available through process.env instead of interpolation. Done means a resolver can read the configured resource value from process.env and the behavior is documented or verified with an equivalent example.

Written by the indexing model from the issue text.