Work around bind:this type mismatch

Address this failure:

    .../halo/src/lib/FracInput.svelte:74:24
    Error: 'input' is possibly 'undefined'. (ts)
            oninput={() => {
                rawValue = input.value
            }}

In practice this is likely harmless, as `bind:this={input}` means that
variable should be bound by the time it emits an input event.
Nonetheless, take the value from the event to avoid closing over `input`.

src/lib/FracInput.svelte

@@ -70,8 +70,8 @@
         value={focused ? rawValue : displayValue}
         bind:this={input}
         {required}
-        oninput={() => {
-            rawValue = input.value
+        oninput={(e) => {
+            rawValue = e.currentTarget.value
         }}
         onfocus={() => {
             focused = true