json-rpc-engine: Wildcard exports -> named exports (#4462)

Using wildcard exports makes it difficult to understand what a package
exports and makes it possible for new exports to get added accidentally.

This commit updates the `@metamask/json-rpc-engine` package to use named
exports instead of wildcard exports, and adds a test to verify non-type
exports following other packages.

packages/json-rpc-engine/src/index.test.ts

@@ -0,0 +1,16 @@
+import * as allExports from '.';
+
+describe('@metamask/json-rpc-engine', () => {
+  it('has expected JavaScript exports', () => {
+    expect(Object.keys(allExports)).toMatchInlineSnapshot(`
+      Array [
+        "createAsyncMiddleware",
+        "createScaffoldMiddleware",
+        "getUniqueId",
+        "createIdRemapMiddleware",
+        "JsonRpcEngine",
+        "mergeMiddleware",
+      ]
+    `);
+  });
+});

packages/json-rpc-engine/src/index.ts

@@ -1,6 +1,18 @@
-export * from './createAsyncMiddleware';
-export * from './createScaffoldMiddleware';
-export * from './getUniqueId';
-export * from './idRemapMiddleware';
-export * from './JsonRpcEngine';
-export * from './mergeMiddleware';
+export type {
+  AsyncJsonRpcEngineNextCallback,
+  AsyncJsonrpcMiddleware,
+} from './createAsyncMiddleware';
+export { createAsyncMiddleware } from './createAsyncMiddleware';
+export { createScaffoldMiddleware } from './createScaffoldMiddleware';
+export { getUniqueId } from './getUniqueId';
+export { createIdRemapMiddleware } from './idRemapMiddleware';
+export type {
+  JsonRpcEngineCallbackError,
+  JsonRpcEngineReturnHandler,
+  JsonRpcEngineNextCallback,
+  JsonRpcEngineEndCallback,
+  JsonRpcMiddleware,
+  JsonRpcNotificationHandler,
+} from './JsonRpcEngine';
+export { JsonRpcEngine } from './JsonRpcEngine';
+export { mergeMiddleware } from './mergeMiddleware';