rustdoc: Properly restore search input placeholder

[fmease]

Fix the search input placeholder literally getting set to the string undefined on blur/defocus.
This was caused by us trying to access an undefined property in the event listener.
To prevent this from regressing again, stop typescript from ignoring the relevant site.

Steps to reproduce the bug fixed in this PR:

1. Focus the search input field by clicking on it and clear the input if necessary
2. Blur/defocus it by clicking somewhere outside of it

---

First bug that would've been caught by TSC if we had had it earlier! Type (quasi-)safety, ahoy! :)

[rustbot]

r? @notriddle

rustbot has assigned @notriddle.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha

[fmease]

To clarify, the minimal fix would be:

  searchState.input.addEventListener("blur", () => {
-     searchState.input.placeholder = searchState.input.origPlaceholder;
+     searchState.input.placeholder = searchState.origPlaceholder;
  });