New rule: `no undefined variable in the DOM`

Motivation

When using a variable (say a props) that can be null or undefined directly in the DOM, it will be rendered as a string "null" or "undefined". Hence, it is probablly never going to be a good thing to use a possibly null or undefined variable directly in the DOM.

Description

The rule would highlight any variable or expression used in the DOM that resolves to a type involving a union type to null or undefined.

Examples

<script lang="ts">
  export let prop: string | undefined = undefined;

  const frameworks = ["svelte", "react"]
</script>

<!-- ✓ GOOD -->
{#if prop}
  {prop}
{/if}

<!-- ✗ BAD -->
{prop}
{frameworks.find(framework => framework === prop)}

Additional comments

No response