Add `targetProductPart` property to allow installing plugin on the frontend part when running in Split Mode (#1563)

[chashnikov]

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targetProductPart property is added to IntelliJPlatformExtension and SplitModeAware. It controls in which part of the IDE the developed plugin is installed. To do that, PrepareSandboxTask generates a properties file which configures the frontend process started from Split Mode. The path to the file is passed via JETBRAINS_CLIENT_PROPERTIES environment variable.

Types of changes

• [ ] Docs change / refactoring / dependency upgrade
• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have included my change in the CHANGELOG.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[chashnikov]

I'm not quite sure about the property name (targetProductPart). If you have a better variant, let's discuss it. Also, BACKEND_AND_FRONTEND entry is too long, I was thinking about using BOTH instead, but decided to keep a more specific name.

I hope in the future we'll be able to automatically determine whether the plugin is compatible with the frontend side, the backend side or both, and this property won't be needed at all.