Mention "import type" in TypeScript FAQ (#5243)

* Mention "import type" in TypeScript FAQ

* Update site/content/faq/500-what-about-typescript-support.md

Co-authored-by: Christian Kaisermann <christian@kaisermann.me>

site/content/faq/500-what-about-typescript-support.md

@@ -10,3 +10,11 @@ To declare the type of a reactive variable in a Svelte template, you should use
 let x: number;
 $: x = count + 1;
 ```
+
+To import a type or interface make sure to use [TypeScript's `type` modifier](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-8.html#type-only-imports-and-export):
+
+```
+import type { SomeInterface } from './SomeFile';
+```
+
+You must use the `type` modifier because `svelte-preprocess` doesn't know whether an import is a type or a value — it only transpiles one file at a time without knowledge of the other files and therefore can't safely erase imports which only contain types without this modifier present.