feat: add gradle support and update project structure

Created by: yaohui-wyh

This PR helps refactoring the IntelliJ plugin project by adding Gradle build system (which is the recommended way for IntelliJ plugin development: https://plugins.jetbrains.com/docs/intellij/gradle-build-system.html).

None of the current plugin features are modified. However, after restructuring it could make future development & release process easier.

Details:

• use Gradle for build configuration instead of IDEA's misc.xml. Developers using IDEA module configuration setups (misc.xml) could just delete the .idea folder and reimport the project
• move java classes into src/main/java source set (for IDEA to reload & re-index the Gradle project)
• move resources into src/main/resources
• update .gitignore
• update development guide in README.md

Tests:

• To run IDE with plugin installed: ./gradlew runIde (checkout available gradle tasks for details)
• To create plugin artifact: ./gradlew buildPlugin

Resources:

• https://github.com/JetBrains/intellij-platform-plugin-template
• https://github.com/JetBrains/intellij-plugins