Harden preview script CSP

[yusufm]

Summary

• add per-render nonces to renderer-owned preview script tags
• remove broad file/self script sources and outbound connect permission from the preview CSP
• add regression coverage for nonce-bearing app scripts and document-supplied file scripts

Validation

• bundle exec pod install
• xcodebuild test -workspace "MacDown 3000.xcworkspace" -scheme MacDown -only-testing:MacDownTests/MPAssetTests -only-testing:MacDownTests/MPRendererEdgeCaseTests/testPreviewRenderIncludesContentSecurityPolicyAndCheckboxToken -only-testing:MacDownTests/MPRendererEdgeCaseTests/testPreviewCSPBlocksDocumentSuppliedFileScripts -destination 'platform=macOS'

Related to security review finding: Preview CSP allows local script execution.