Fixed bug with slot props variables

[primos63]

#2586

Fixes issue when declaring and initializing a variable used in a slot.
The variable is hoisted and subsequently not added to filtered_declarations. The return statement for the component instance would be
return { $$slots, $$scope };

when it should be
return { worldText, $$slots, $$scope };

Test component-slot-let-e will fail when worldText is not part of the return object.

1) runtime
      component-slot-let-e :

     AssertionError [ERR_ASSERTION]: '<div><p>undefined</p></div>' deepEqual '<div><p>foo</p></div>'
     + expected - actual

     -<div><p>undefined</p></div>
     +<div><p>foo</p></div>
     
     at Function.assert.htmlEqual (test/helpers.js:141:10)
     at Promise.resolve.then (test/runtime/index.js:155:14)

Other slot tests did not fail because the on:click handler caused the variable to be not be hoisted.

[Rich-Harris]

Thank you! There's actually a slightly more efficient fix, which is to re-use the hoisted value (and not shadow it) — I've opened #3124 (which is a branch off this PR), so will close this