Refactor Dialog abstraction (#90)

This PR overhauls the Dialog abstractions in `studio-ui` to be more
flexible for a wider variety of use cases, and adds a placeholder
"Import Asset" sub-dialog to the Media Library dialog.

utility/src/index.ts

@@ -1,3 +1,4 @@
+export * from "./lib/array";
 export * from "./lib/assert";
 export * from "./lib/exists";
 export * from "./lib/match";

utility/src/lib/array.ts

@@ -0,0 +1,20 @@
+export type ArrayOrSingle<T> = T | T[];
+
+export function asArray<T>(value: ArrayOrSingle<T> | Iterable<T>): T[] {
+	if (Array.isArray(value))
+		return value;
+
+	if (isIterable<T>(value))
+		return Array.from(value);
+
+	return [value];
+}
+
+export function isIterable<T = unknown>(value: unknown): value is Iterable<T> {
+	return (
+		value != null
+		&& typeof value === "object"
+		&& Symbol.iterator in value
+		&& typeof value[Symbol.iterator] === "function"
+	)
+}