feat(prefer-svelte-reactivity): reporting returned variables

[marekdedic]

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Latest commit: 5774d2f

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eslint-plugin-svelte	Minor

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[baseballyama]

Why should we be forced to use SvelteSet or SvelteMap even when the values aren’t changing?

[marekdedic]

Firstly, this PR is part of a wider effort to move the rule to better detect when variables are "encapsulated" inside a function/class and not report them - this PR is an off-shoot of #1287

But this should make sense even on its own anyway - but I think it does. Example

fileA:

function makeSet() {
  const a = new Set();
  // Populate the set etc.
  return a;
}

export makeSet;

fileB:

<script>
  import { makeSet } from "fileA";

  const a = makeSet();
</script>

<!-- Print the set here -->

<button onclick={a.add(42);}>Click me!</button>

In this example, this PR reports the incorrect usage of Set. I hope to pair it with the rest of #1287 to ignore all encapsulated uses by default so that we don't get too many FPs.