Move flowchart demo from JointJS+ to @joint/core

[Geliogabalus]

Summary

• Migrates the flowchart JS demo from @joint/plus (private registry) to @joint/core (public npm)
• Removes .npmrc and private registry configuration, eliminating the need for an access token
• Simplifies setup instructions in README files accordingly
• Removes JointJS+ badge links from README headings

Test plan

• Run npm install in flowchart/js/ and verify it resolves @joint/core from public npm without auth
• Run npm run dev and verify the flowchart demo renders correctly
• Confirm no references to @joint/plus remain in flowchart/js/

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