OverrideProperties: Fix autocomplete in the type (#712)

source/override-properties.d.ts

@@ -25,5 +25,12 @@ type Fizz = OverrideProperties<Foo, {b: number; c: number}>
 */
 export type OverrideProperties<
 	TOriginal,
-	TOverride extends {[Key in keyof TOverride]: Key extends keyof TOriginal ? TOverride[Key] : never},
+	// This first bit where we use `Partial` is to enable autocomplete
+	// and the second bit with the mapped type is what enforces that we don't try
+	// to override properties that doesn't exist in the original type.
+	TOverride extends Partial<Record<keyof TOriginal, unknown>> & {
+		[Key in keyof TOverride]: Key extends keyof TOriginal
+			? TOverride[Key]
+			: never;
+	},
 > = Merge<TOriginal, TOverride>;