[fix] warning on reactive use of module const (#6606)

[OJFord]

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

NB there are two warnings, but I also see them on master:

[...]/svelte/src/compiler/compile/create_module.ts
  100:3   warning  Forbidden non-null assertion  @typescript-eslint/no-non-null-assertion
  100:36  warning  Forbidden non-null assertion  @typescript-eslint/no-non-null-assertion

---

Fixes #6606.

This generated a warning:

<script context="module">
  export const foo = 123;
</script>

<script>
  $: console.log(foo);
</script>

while this did not:

<script>
  const foo = 123;
  $: console.log(foo);
</script>

We don't need a warning to tell us that we will not be reactive to a
variable that isn't going to change anyway; regardless of whether or not
it's in a module.

(Maybe there should be a 'no reactive variable used' warning, but this
also happens in cases where other mutable variables are used; such as a
shared const formatter function, formatting a prop.)

[OJFord]

Thanks!