Extract helper types out of `create-store.ts`

Per feedback at
https://github.com/hypothesis/client/pull/4944#pullrequestreview-1178131884, try
to make `create-store.ts` easier to scan by moving some of the typing-related
utilities into a separate module.

src/sidebar/store/create-store.ts

@@ -4,11 +4,7 @@ import * as redux from 'redux';
 import thunk from 'redux-thunk';
 
 import { immutable } from '../util/immutable';
-
-/** Helper that strips the first argument from a function type. */
-type OmitFirstArg<F> = F extends (x: any, ...args: infer P) => infer R
-  ? (...args: P) => R
-  : never;
+import type { OmitFirstArg, TupleToIntersection } from './type-utils';
 
 /**
  * Helper that converts an object of selector functions, which take a `state`
@@ -116,21 +112,6 @@ function assignOnce<T extends object, U extends object>(target: T, source: U) {
   return Object.assign(target, source);
 }
 
-type MapContravariant<T> = { [K in keyof T]: (x: T[K]) => void };
-
-/**
- * Utility that turns a tuple type `[A, B, C]` into an intersection `A & B & C`.
- *
- * The implementation is magic adapted from
- * https://github.com/microsoft/TypeScript/issues/28323. Roughly speaking it
- * works by computing a type that could be assigned to any position in the
- * tuple, which must be the intersection of all the tuple element types.
- */
-type TupleToIntersection<
-  T,
-  Temp extends Record<number, unknown> = MapContravariant<T>
-> = Temp[number] extends (x: infer U) => unknown ? U : never;
-
 /**
  * Create a Redux store from a set of modules.
  *

src/sidebar/store/type-utils.ts

+type MapContravariant<T> = { [K in keyof T]: (x: T[K]) => void };
+
+/**
+ * Utility that turns a tuple type `[A, B, C]` into an intersection `A & B & C`.
+ *
+ * The implementation is magic adapted from
+ * https://github.com/microsoft/TypeScript/issues/28323. Roughly speaking it
+ * works by computing a type that could be assigned to any position in the
+ * tuple, which must be the intersection of all the tuple element types.
+ */
+export type TupleToIntersection<
+  T,
+  Temp extends Record<number, unknown> = MapContravariant<T>
+> = Temp[number] extends (x: infer U) => unknown ? U : never;
+
+/**
+ * Helper that strips the first argument from a function type.
+ *
+ * This maps a type like `(a: T1, b: T2, ...) => Result` to `(b: T2, ...) => Result`.
+ */
+export type OmitFirstArg<F> = F extends (x: any, ...args: infer P) => infer R
+  ? (...args: P) => R
+  : never;