Set prop's initialised property based on declarator

[pshrmn]

Currently, a variable's initialised value is set based on the init property of a VariableDeclaration. However, the init property exists on VariableDeclarators, not VariableDeclarations, so initialised is always false.

<script>
  export let myVar = 3;
  // internally, myVar.initialised = false
</script>

This PR sets initialised based on the init value of a VariableDeclarator.

This change allows a prop to be explicitly initialized to undefined without receiving a warning.

<script>
  export let thing = undefined; // will not warn if not provided
  export let other; // will warn if not provided
</script>

[Rich-Harris]

Thank you! I saw #2637 (which is basically identical fix) and merged it before I saw this, so I'll close this PR