Type-only imports

[mjarr]

Currently .guard.ts files import needed types using normal imports, which results in ts(1371) and ts(1444) errors in my environment (svelte). Making the imports type-only would solve this issue.

[rhys-vdw]

I hadn't heard of this feature. Would this change the minimum TypeScript version?

[mjarr]

They were introduced in TypeScript 3.8.
Seems like all that needs to be done to enable them is to add isTypeOnly: true to objects pushed to structures in outFile.addImportDeclarations.

[rhys-vdw]

Okay cool, I think it's reasonably to upgrade the minimum version to 3.8. Note that this would not always be appropriate due to the way that classes are checked. See discussion in #162.