Update MultiFieldSelect to accept a defaultValue

This changes the multi select control to accept defaultValues. As these are passed in as strings (flattened arrays in the form of "1, 2, 3") they need to be split into an array to be accepted by the control.

packages/client/src/components/app/forms/MultiFieldSelect.svelte

@@ -20,6 +20,7 @@
   let fieldSchema
 
   $: flatOptions = optionsSource == null || optionsSource === "schema"
+  $: expandedValue = expand(fieldState?.value)
   $: options = getOptions(
     optionsSource,
     fieldSchema,
@@ -28,6 +29,18 @@
     valueColumn,
     customOptions
   )
+
+  const expand = values => {
+    if (!values) {
+      return []
+    }
+
+    if (Array.isArray(values)) {
+      return values
+    }
+
+    return values.split(",")
+  }
 </script>
 
 <Field
@@ -43,7 +56,7 @@
 >
   {#if fieldState}
     <CoreMultiselect
-      value={fieldState.value || []}
+      value={expandedValue}
       error={fieldState.error}
       getOptionLabel={flatOptions ? x => x : x => x.label}
       getOptionValue={flatOptions ? x => x : x => x.value}