demo: add codiconDecoration proposed api for read-only schemes

[shouples]

Summary of Changes

• https://code.visualstudio.com/api/advanced-topics/using-proposed-api

Optional: Click-testing instructions

1. Open VS Code Insiders and run this extension in normal dev mode
2. Open one of our read-only document types (message preview, schema, Flink statement)
3. Note the document tabs now show a lock codicon with a Read-only tooltip suffix

Optional: Any additional details or context that should be provided?

Pull request checklist

Please check if your PR fulfills the following (if applicable):

Tests

• Added new
• Updated existing
• Deleted existing

Release notes

• Does anything in this PR need to be mentioned in the user-facing CHANGELOG?

[sonarqube-confluent]

Analysis Details

0 Issues

• 0 Bugs
• 0 Vulnerabilities
• 0 Code Smells

Coverage and Duplications

• 100.00% Coverage (73.30% Estimated after merge)
• No duplication information (0.50% Estimated after merge)

Project ID: vscode

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