docs: add the snack demo and document its upkeep

[JoshuaPariona]

Summary

Adds snack/, the source of the published
Expo Snack, and links
it from the README so the module can be tried from a browser or Expo Go with
nothing installed. The Snack exists because 1.1.0 made the native module
optional: react-native-webview already ships inside Expo Go, so the view runs
there.

The demo is deliberately small next to example/ — one screen, one document,
four highlighters — and it is what the README, the React Native Directory entry
and future answers point at.

Why the source lives in the repo: Snack has no git integration to rely on, so
the folder is the reviewed, formatted, versioned copy and the Snack is a mirror
of it. snack/README.md carries the procedure, and docs/VERSION-UPDATE.md
now lists snack/package.json among the places a release has to touch —
a stale pin there demos an older library than the one people install.

Also here:

• react-native-libraries-entry.json, our React Native Directory entry, now
  documented in CONTRIBUTING.md (the live copy lives in that project's repo;
  this is the local original). It is already open as
  react-native-community/directory#2828.
• yarn lint now covers snack/ too: the ESLint config always matched those
  files, but expo-module lint only ever passed src to ESLint.
• docs/VERSION-UPDATE.md gained the cosmetic references and the other version
  strings that drift the same way — the react-native-webview range duplicated
  in four places, and the vendored rangy@1.3.2 directory name that appears in
  nine.

Type of change

• fix — bug fix (no public API change)
• feat — new backward-compatible API
• Breaking change to the public API or the RN ↔ WebView bridge contract
• docs / chore / ci / test — no runtime change

Nothing under src/, ios/ or android/ is touched, and snack/ stays out of
the published package: files in package.json is a whitelist, so
npm pack --dry-run ships zero files from it.

How was this tested?

yarn lint (now including the five files in snack/), yarn typecheck and
yarn test (151 tests) pass.

The demo itself was run on Android through a prebuilt app that installs the
published 1.1.0 from npm: select a passage, apply a highlighter from the native
menu, clear everything, then undo — the highlights come back from the
serialized payload, and the undo button disables itself when the history runs
out.

• Android (full interaction loop)
• iOS — not exercised by hand; the simulator takes no input from the CLI
  without accessibility permission

Checklist

• yarn lint, yarn typecheck, and yarn test all pass.
• Public API changes are documented — there are none.
• Bridge changes keep all three sides in sync — no bridge change here.
• CHANGELOG.md updated under ## [Unreleased] — not applicable: no
  runtime change ships to consumers from this PR.
• I did not bump the version.