Add preinstall script to try to fix eas build issues

EAS build complains that it can't scandir ../web/dist, even though that was already built on the Github runner.
But EAS build --local is to simulat e an EAS cloud run. It doesn't run in the current directory, rather it runs in a temp directory /tmp/runner/eas-build-local-nodejs/b5b2f922-e43d-427b-aa00-d516325d63b0/build/packages/mobile

(As opposed to in /home/runner/work/BloomReader-Lite/BloomReader-Lite/packages/mobile)

packages/mobile/TODO.md

@@ -31,3 +31,5 @@ How packages/mobile consumes packages/web
 
 It'd be nice to be able to run multiple variants on the same device.
 Follow the instructions here: https://docs.expo.dev/build-reference/variants/
+
+If this yarn preinstall works (allows alpha/beta builds to succeed), then you should refactor build-local.yml to cut out the steps that build shared and web.

packages/mobile/package.json

@@ -10,6 +10,7 @@
  "web": "expo start --web",
  "build:ios": "yarn && eas build --platform ios --profile development",
  "build:ios:prod": "yarn && eas build --platform ios --profile production",
+ "preinstall": "cd ../shared && yarn && yarn build && cd ../web && yarn && yarn build && cd ../mobile",
  "postinstallComments": "// Runs automatically after running 'yarn' or 'yarn install'",
  "postinstall": "yarn copy:web",
  "copy:web": "ts-node scripts/copyWebDist.ts"