Use Gradle to build the Courier schema definition plugin

[rbalent-coursera]

Description

Use Gradle to build the Courier schema definition plugin.

The configuration files are based on the intellij-platform-plugin-template.

How to release a new version of the plugin

1. Check out this PR
2. See the latest commit to this PR. Only 4 lines need to be updated: cb89f9f (#89)
3. Increase the plugin version and check the intellij-platform-plugin-template for the new platform versions you need to put there.
4. ./gradlew build
5. This will generate the build/libs/Courier\ schema\ language-1.6.x.jar file.
6. Replace the idea-plugin.jar with this file. Like here: Update plugin for IntelliJ IDEA 2025.1 #92