This removes our restriction around using an await expression at the top level of a file when the following conditions are met:

• The containing file is an external module (or --isolatedModules is provided)
• The --target is >= ScriptTarget.ES2017 (minimum version required for await keyword)
• The --module is either esnext or system.

This is not currently supported for earlier script targets (e.g., ES2016, ES2015, ES5, ES3) as the system transform currently happens after async functions and generators have been transformed, so it is too late to transform the async function that is created as part of the system module transform. This also ensures it is consistent with --target es5 --module esnext, as there would be no way to down-level the await in that context either.

Fixes #25988, #32793