deps: Use Jest 26, the latest, with a resolutions hack.

We already declare a direct dependency on Jest 26 (and have done
since fb23341), but it doesn't get pulled in. Empirically, that
seems to be because `jest-expo` brings in Jest 25.

We know we'll soon want Jest 26 for a few things:

- The "modern" implementation of fake timers [1], for zulip#4165.

- The `selectProjects` CLI argument [2], for testing Android
  codepaths in a nice way (later in this series).

But we've *also* realized that it'd be nice to keep `jest-expo`, to
use the `jest-expo/ios` and `jest-expo/android` presets (which we'll
do later in this series). We hope we can end the pattern of adding
and removing `jest-expo` again and again:

62621ef jest: Add `jest-expo` preset, to be used in the next commit.
347aa96 jest: Back out `jest-expo` preset, for now.
c4fca9d jest: Add and use `jest-expo` preset, again.

This is a hack because the `resolutions.jest-expo/jest` range
`^26.4.1` is incompatible with the original range that `jest-expo`
declares; that's `^25.2.0`. We believe this should get us a warning
from Yarn; according to a doc [3], "You will receive a warning if
your resolution version or range is not compatible with the original
version range." I haven't seen a warning like that, even after
removing node_modules and running `yarn`; this seems like a Yarn
bug.

I've opened expo/expo#12735 to bump Jest in the `jest-expo` project;
this hack should hopefully be fine while we wait for that.

[1] https://github.com/facebook/jest/blob/master/CHANGELOG.md#2600
[2] https://github.com/facebook/jest/blob/master/CHANGELOG.md#2610
[3] https://classic.yarnpkg.com/en/docs/selective-version-resolutions/#toc-tips-tricks

package.json

@@ -139,5 +139,8 @@
     "rollup": "^2.26.5",
     "typescript": "~3.8.3",
     "yarn-deduplicate": "^3.0.0"
+  },
+  "resolutions": {
+    "jest-expo/jest": "^26.4.1"
   }
 }