Allow running the application directly with './gradlew run' command

[pdecat]

This PRs simplifies running the application directly from a fresh clone without having to package the application using the gradle Application Plugin: https://docs.gradle.org/current/userguide/application_plugin.html

Without this change, the ./gradlew run command would fail with the following error message:

# ./gradlew run
> Task :run FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':run'.
> No main class specified and classpath is not an executable jar.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.4/userguide/command_line_interface.html#sec:command_line_warnings

BUILD FAILED in 631ms
3 actionable tasks: 1 executed, 2 up-to-date

[kaikramer]

Thanks for the PR!