refactor(generation-transformer): use file templates for JS resolvers

[atierian]

Description of changes

Note

No functional changes

• moves string defined JS resolvers to JavaScript file templates using [[KEY]] marker for template substitution.
• adds copy-js-resolver-templates to move JS templates to /lib as part of build script.

Why use [[KEY]] as the marker?
This marker format satisfies two key goals:

1. Minimal risk of overwriting previously substituted values, e.g. the value of [[KEY1]] containing "[[KEY2]]".
2. Not break syntax highlighting in the JS file templates. The motivation behind moving to file templates is to improve maintainability of the resolver functions, for which syntax highlighting plays a vital role.

I'm satisfied that [[KEY]] is the appropriate format here. That said if you have an alternative in mind that satisfies both 1. and 2., and has other benefits, I'm happy to consider it.

CDK / CloudFormation Parameters Changed

N/A

Issue #, if available

N/A

Description of how you validated changes

• CI PR workflow
• locally run generation E2E tests

Checklist

• PR description included
• yarn test passes
• E2E test run linked
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Any CDK or CloudFormation parameter changes are called out explicitly

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.