Add missing files entry to addon package

[gilest]

Tested the dist branch in a local project and noticed that _app_ files are missing from the packaged output. Example build

This causes the addon to break app builds

Can also be reproduced by running pnpm pack

Before

➜  ember-cli-flash git:(master) npm pack

> ember-cli-flash@4.0.0 prepack
> rollup --config


 → dist...
created dist in 541ms
npm notice
npm notice 📦  ember-cli-flash@4.0.0
npm notice === Tarball Contents ===
npm notice 5.9kB  .eslintcache
npm notice 101B   .eslintignore
npm notice 853B   .eslintrc.cjs
npm notice 101B   .prettierignore
npm notice 109B   .prettierrc.cjs
npm notice 63B    .template-lintrc.cjs
npm notice 1.1kB  LICENSE.md
npm notice 20.5kB README.md
npm notice 114B   addon-main.cjs
npm notice 352B   babel.config.json
npm notice 483B   declarations/flash/object.d.ts
npm notice 1.3kB  declarations/services/flash-messages.d.ts
npm notice 3.1kB  package.json
npm notice 2.6kB  rollup.config.mjs
npm notice 0B     src/.gitkeep
npm notice 535B   src/components/flash-message.hbs
npm notice 3.3kB  src/components/flash-message.js
npm notice 3.1kB  src/flash/object.js
npm notice 4.7kB  src/services/flash-messages.js
npm notice 232B   src/test-support.js
npm notice 572B   src/utils/computed.js
npm notice 362B   src/utils/flash-message-options.js
npm notice 421B   src/utils/get-with-default.js
npm notice 309B   src/utils/object-compact.js
npm notice 242B   src/utils/object-only.js
npm notice 242B   src/utils/object-without.js
npm notice === Tarball Details ===
npm notice name:          ember-cli-flash
npm notice version:       4.0.0
npm notice filename:      ember-cli-flash-4.0.0.tgz
npm notice package size:  14.6 kB
npm notice unpacked size: 50.6 kB
npm notice shasum:        6cba5af473a4b63228cd751c7b2d2c6426140672
npm notice integrity:     sha512-2F6de7CWsd15n[...]D07E/kUfXzVvQ==
npm notice total files:   26
npm notice
ember-cli-flash-4.0.0.tgz

After

➜  ember-cli-flash git:(fix/add-files-package-entry) npm pack

> ember-cli-flash@4.0.0 prepack
> rollup --config


 → dist...
created dist in 517ms
npm notice
npm notice 📦  ember-cli-flash@4.0.0
npm notice === Tarball Contents ===
npm notice 1.1kB  LICENSE.md
npm notice 20.5kB README.md
npm notice 114B   addon-main.cjs
npm notice 483B   declarations/flash/object.d.ts
npm notice 1.3kB  declarations/services/flash-messages.d.ts
npm notice 68B    dist/_app_/components/flash-message.js
npm notice 67B    dist/_app_/services/flash-messages.js
npm notice 2.1kB  dist/_rollupPluginBabelHelpers-e795903d.js
npm notice 181B   dist/_rollupPluginBabelHelpers-e795903d.js.map
npm notice 6.6kB  dist/components/flash-message.js
npm notice 10.9kB dist/components/flash-message.js.map
npm notice 3.7kB  dist/flash/object.js
npm notice 7.8kB  dist/flash/object.js.map
npm notice 5.7kB  dist/services/flash-messages.js
npm notice 11.7kB dist/services/flash-messages.js.map
npm notice 304B   dist/test-support.js
npm notice 628B   dist/test-support.js.map
npm notice 615B   dist/utils/computed.js
npm notice 1.1kB  dist/utils/computed.js.map
npm notice 460B   dist/utils/flash-message-options.js
npm notice 1.1kB  dist/utils/flash-message-options.js.map
npm notice 488B   dist/utils/get-with-default.js
npm notice 896B   dist/utils/get-with-default.js.map
npm notice 372B   dist/utils/object-compact.js
npm notice 848B   dist/utils/object-compact.js.map
npm notice 303B   dist/utils/object-only.js
npm notice 774B   dist/utils/object-only.js.map
npm notice 306B   dist/utils/object-without.js
npm notice 780B   dist/utils/object-without.js.map
npm notice 3.2kB  package.json
npm notice === Tarball Details ===
npm notice name:          ember-cli-flash
npm notice version:       4.0.0
npm notice filename:      ember-cli-flash-4.0.0.tgz
npm notice package size:  22.5 kB
npm notice unpacked size: 84.4 kB
npm notice shasum:        df61aac6c84659483a138292d0980592a77d8d2e
npm notice integrity:     sha512-wl6PwoHH+h6Yj[...]XgCXcHl5oeoUA==
npm notice total files:   30
npm notice
ember-cli-flash-4.0.0.tgz

[gilest]

@NullVoxPopuli please merge this fix. It's blocking release

After that I'd like to test with the push-dist branch but otherwise we should be ready for release