Preserve all typescript imports, not only .svelte

[geakstr]

Hi! Thanks for the project. Library already preserves all .svelte imports. This PR change behaviour to preserve all imports, because imported variables can be not only svelte components but also values which svelte template actually uses. Example:

<script lang="ts">
  import { writable } from "svelte/store";
  import { count } from "./stores";
  import Counter from "./Counter.svelte";

  export let name: number;
  export let age: number;
</script>

<h1>Hello {name} ({age})!</h1>
<p>
  <Counter />
  <Counter value={1}>Counter 1</Counter>
  <Counter value={$count} step={3}>Counter 2</Counter>
</p>

Without the fix compiler throws:

(!) svelte plugin: 'count' is not defined
src/App.svelte
   <Counter />
  <Counter value={1}>Counter 1</Counter>   
  <Counter value={$count} step={3}>Counter 2</Counter>
                  ^
</p>

Relates to:

• svelte plugin: <Component> is not defined #4
• Should be possible to force import of "unused" modules in case there are side-effects microsoft/TypeScript#9191