Use type-only import for better verbatimModuleSyntax support

[alebelcor]

Hi, I have this weird TypeScript issue where—even though I have excluded node_modules in my tsconfig.json—I get a type error from "htmr" when using tsc.

I'm using verbatimModuleSyntax (documentation) to simplify import elision. The setting comes down to ensuring you use the type modifier when importing types. So I did that in src/types.ts.

I don't think it'd have any negative impact since yarn typecheck ran successfully. But feel free to let me know otherwise.

Thanks for considering!

[lakyadam]

Looks good,. Can we get this merged?

@dmfrancisco @greggb @Arcath