(svelte2tsx) wrap template with callback

[jasonlyu123]

fix for #310 and better union type for template

<script>
let a: 'a' | 'b' = 'a';

const handler = () => {
  a = 'b'
}
</script>

{ a } <!-- a will have type 'a' | 'b' now -->

[jasonlyu123]

Since it'll affect the "most-discussed-problem", the reactive declaration, This will be blocked until a better solution for both problems.

[jasonlyu123]

Umm... Why are there so many merging conflicts when the edit is on the different line lol