add no-reactive-functions rule

[tivac]

📖 Rule Details

This rule reports whenever a function is defined in a reactive statement. This isn't necessary, as each time the function is executed it'll already have access to the latest values necessary. Redefining the function in the reactive statement is just a waste of CPU cycles.

<script>
  /* eslint svelte/no-reactive-functions: "error" */

  /* ✓ GOOD */
  const arrowFn = () => { /* ... */ }
  const func = function() { /* ... */ }
  
  /* ✗ BAD */
  $: arrowFn = () => { /* ... */ }
  $: func = function() { /* ... */ }
</script>

[ota-meshi]

LGTM! Thank you very much!