Introduce menu entry support

Summary:
[interesting] since it shows how Flipper APIs are exposed through sandy. However, the next diff is a much simpler example of that

This diff adds support for adding menu entries for sandy plugin (renamed keyboard actions to menus, as it always creates a menu entry, but not necessarily a keyboard shortcut)

```

  client.addMenuEntry(
    // custom entry
    {
      label: 'Reset Selection',
      topLevelMenu: 'Edit',
      handler: () => {
        selectedID.set(null);
      },
    },
    // based on built-in action (sets standard label, shortcut)
    {
      action: 'createPaste',
      handler: () => {
        console.log('creating paste');
      },
    },
  );
```

Most of this diff is introducing the concept of FlipperUtils, a set of static Flipper methods (not related to a device or client) that can be used from Sandy. This will for example be used to implement things as `createPaste` as well

Reviewed By: nikoant

Differential Revision: D22766990

fbshipit-source-id: ce90af3b700e6c3d9a779a3bab4673ba356f3933

desktop/flipper-plugin/src/plugin/MenuEntry.tsx

@@ -0,0 +1,66 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+export type DefaultKeyboardAction = 'clear' | 'goToBottom' | 'createPaste';
+export type TopLevelMenu = 'Edit' | 'View' | 'Window' | 'Help';
+
+export type MenuEntry = BuiltInMenuEntry | CustomMenuEntry;
+
+export type NormalizedMenuEntry = {
+  label: string;
+  accelerator?: string;
+  topLevelMenu: TopLevelMenu;
+  handler: () => void;
+  action: string;
+};
+
+export type CustomMenuEntry = {
+  label: string;
+  accelerator?: string;
+  topLevelMenu: TopLevelMenu;
+  handler: () => void;
+};
+
+export type BuiltInMenuEntry = {
+  action: keyof typeof buildInMenuEntries;
+  handler: () => void;
+};
+
+export const buildInMenuEntries = {
+  clear: {
+    label: 'Clear',
+    accelerator: 'CmdOrCtrl+K',
+    topLevelMenu: 'View',
+    action: 'clear',
+  },
+  goToBottom: {
+    label: 'Go To Bottom',
+    accelerator: 'CmdOrCtrl+B',
+    topLevelMenu: 'View',
+    action: 'goToBottom',
+  },
+  createPaste: {
+    label: 'Create Paste',
+    topLevelMenu: 'Edit',
+    action: 'createPaste',
+  },
+} as const;
+
+export function normalizeMenuEntry(entry: MenuEntry): NormalizedMenuEntry;
+export function normalizeMenuEntry(entry: any): NormalizedMenuEntry {
+  const builtInEntry:
+    | NormalizedMenuEntry
+    | undefined = (buildInMenuEntries as any)[entry.action];
+  return builtInEntry
+    ? {...builtInEntry, ...entry}
+    : {
+        ...entry,
+        action: entry.action || entry.label,
+      };
+}