Svelte files that use typescript should allow typescript in inline handlers in the template section.

[PeytonHanel]

Describe the problem

I use typescript in Svelte by creating a script tag like this <script lang='ts'>, but unintuitively this doesn't allow typescript to be used in the template section with inline handlers. Here is an example:

<button on:click={() => {
          let n: number = 1; // <--- underlines with an error squiggly 
}}>Accept</button>

But even more unintuitively, if I'm accessing a property that might not be there, for example, the parent form from the event, I can hide the red squiggly error with // @ts-ignore.

<form
    action='?/acceptFollowRequest'
    method='post'
  >
    <button class='btn-small' on:click={(event) => {
      // @ts-ignore
      console.log(event.target.form);  // <--- would normally red underline
    }}>Accept</button>
  </form> 

Describe the proposed solution

Any code inline in the template section should match the <script lang='ts'> declaration at the top of the file.

Alternatives considered

The current alternative is to just use javascript, or instead write out a function in the <script lang='ts'> tag. These are both ok solutions but I feel that my feature request would make Svelte more consistent and intuitive in the long run.

Importance

nice to have

[gtm-nayan]

Duplicate of #4701