Create Plugin: Externalise unstable entrypoints

[jackw]

What this PR does / why we need it:
This PR adds unstable entrypoints to the externals webpack config.

We need to merged and released before we merge the related Grafana PR: grafana/grafana/pull/97080

Which issue(s) this PR fixes:

Related grafana/grafana-community-team/issues/327

Special notes for your reviewer:

📦 Published PR as canary version: Canary Versions

✨ Test out this PR locally via:

npm install @grafana/create-plugin@5.18.4-canary.1580.13626560591.0
npm install @grafana/eslint-plugin-plugins@0.2.3-canary.1580.13626560591.0
npm install @grafana/plugin-types-bundler@0.2.7-canary.1580.13626560591.0
# or 
yarn add @grafana/create-plugin@5.18.4-canary.1580.13626560591.0
yarn add @grafana/eslint-plugin-plugins@0.2.3-canary.1580.13626560591.0
yarn add @grafana/plugin-types-bundler@0.2.7-canary.1580.13626560591.0

[github-actions]

Hello! 👋 This repository uses Auto for releasing packages using PR labels.

✨ This PR can be merged and will trigger a new patch release.
NOTE: When merging a PR with the release label please avoid merging another PR. For further information see here.

[hugohaggmark]

LGTM 🎉

[leventebalogh]

Nice 👏

[grafana-plugins-platform-bot]

🚀 PR was released in @grafana/create-plugin@5.18.4, @grafana/eslint-plugin-plugins@0.2.3, @grafana/plugin-types-bundler@0.2.7 🚀