Create new mode to automatically and periodically archive browser bookmarks

[machawk1]

Per @weirdalsuperfan in #542:

we often don't realize we wanted to save a page until we go back to it and realize it's gone. So we don't always save pages we visit.

[machawk1]

On macOS, Google Chrome bookmarks are stored in:

/Users/(USERNAME)/Library/Application Support/Google/Chrome/Default/bookmarks

...as a JSON file, e.g.,

{
   "checksum": "ab7499b01ccc8b4442b1a3dfbaa56d54",
   "roots": {
      "bookmark_bar": {
         "children": [ {
            "date_added": "13068834331760000",
            "guid": "33fd2e0d-00f6-fbda-bc5b-b08b0fe11be3",
            "id": "5",
            "meta_info": {
               "last_visited_desktop": "13178836817449745"
            },
            "name": "",
            "type": "url",
            "url": "http://example.com"
         }, {
            "children": [ {
               "date_added": "13239494202221762",
               "guid": "596b59c6-4eb3-40e5-be2d-41578d94d6ae",
               "id": "7",
               "name": "Lorem Ipsum",
               "type": "url",
               "url": "https://example.com/longerurl.php"
            }, {
               "date_added": "13227737970128135",
               "guid": "d2d396d6-d1a6-4d03-a9d1-78ec3d1cef85",
               "id": "8",
               "name": "Another URL",
               "type": "url",
               "url": "https://example.com/longerurl.php2"
            }],
            "date_added": "13267547399822535",
            "date_modified": "13267547399822535",
            "guid": "16b4f6d1-45b4-4f8a-a4ec-fccc6aa14275",
            "id": "6",
            "name": "myfolder",
            "type": "folder"
         }],
         "date_added": "13267303887326366",
         "date_modified": "13277255203545711",
         "guid": "00000000-0000-4000-a000-000000000002",
         "id": "1",
         "name": "Bookmarks Bar",
         "type": "folder"
      },
...