remove unreferenced variable from initial context

[tanhauhau]

if you look at the output of the following:

<script>
  let foo;
  function unreferenced () {
    foo = 1;
  }
</script>
<button on:click={() => foo = 2}></button>

REPL

you would notice instance is returning [foo, unreferenced, click_handler], unreferenced is part of the array, but it is not referenced anywhere in the fragment.

this change removes it:

- return [foo, unreferenced, click_handler];
+ return [foo, click_handler];

Additional changes

• the calculation of initial_context is shifted from src/compiler/compile/render_dom/index.ts to src/compiler/compile/render_dom/Renderer.ts closer to the logic of sorting of context