ci: announce SDK releases on Discord

[bendersej]

Background

SDK releases are announced in the SimplePDF Discord. Changesets already writes each package's CHANGELOG and GitHub release; this posts the same notes to the releases channel when a release goes out.

Changes

• scripts/announce-release.mjs posts one message per published package with changes, built from the section changesets wrote into its CHANGELOG:
  • notes grouped under Breaking changes / What's new / Fixes and improvements, without commit hashes or "Updated dependencies" lines
  • a section with a single entry reads as a paragraph
  • a Get started field: the script tag for packages with an unpkg build, plus npm install
  • Release notes · npm · Docs links
  • brand blue, red when the release has breaking changes
  • mentions disabled
• A release that only updated dependencies gets no message of its own: it is listed under "Also released" in the first one.
• Notes over Discord's limit are cut at an entry boundary, in order, with a link to the full release notes.
• The release workflow runs it after changesets/action, only when something was published, with the DISCORD_RELEASES_WEBHOOK_URL secret.
• Without the secret, or with DRY_RUN=1, it prints the messages instead of posting them.
• Exit codes: 2 invalid published packages, 3 missing CHANGELOG section, 4 Discord rejected a message.
• node:test suite (5 tests, no new dependency), run by a new Scripts workflow on PRs touching scripts/.

Notes

• Dry-run against the real changeset version output for the upcoming @simplepdf/web-embed-pdf 1.9.0 builds a valid message (1,232-character description).
• Merge before the next version PR so that release is announced.