feat: support top-level await

[aaronadamsCA]

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Description

Adds support for top-level await in a GraphQL config file. Includes tests.

This is good as more libraries go async-only. For us, we needed to sign a JWT in an authorization header, and we preferred to use jose, which is async only.

Fixes #1628.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

I added new tests that run in async mode only. They're all passing, as are all existing tests.

Checklist:

• I have followed the CONTRIBUTING doc and the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Further comments

I avoided any dependency changes, but when you do eventually upgrade cosmiconfig to latest, you could replace the direct usage of jiti with cosmiconfig-typescript-loader. Its latest version provides both async and sync loaders.