Make API for exporting JS content more explicit

[RunDevelopment]

This PR brings the improvements to the export function from #4258. Having this change will make implementing another PR easier, so I wanted to make a separate PR for them now.

Change:

• Instead of trying to detect what kind of expression is being exported from the string value of the JS expression, an enum is used to explicitly spell out what types of expressions the export function supports. It is the responsibility of the caller to ensure the string matches the contract laid out by the ExportJs type. The makes the export function a lot more explicit and easier to change.

This PR is purely internal refactoring and doesn't affect the generated JS in any way.