Fix types (#266) (#267)

With a small change we should be able to make the types we already
have apply to the `./helpers/svg-jar` module.

Not addressed here:

- The types claim the helper to be a class-based helper
- Do we want people to be importing from `./helpers/svg-jar` or do
  we want a re-export
- Do we want to restrict what can be imported via package.json's
  `exports` field

...etc

Basically keeping things working as it is

packages/ember-svg-jar/addon/template-registry.d.ts

@@ -1,4 +1,4 @@
-import type SvgJar from './types';
+import type SvgJar from './helpers/svg-jar';
 
 export default interface EmberSvgJarRegistry {
   'svg-jar': typeof SvgJar;