jmp's test plugin for paperweight-userdev development

(also serves as an example until more thorough documentation is created)

• build.gradle.kts and settings.gradle.kts both contain important configuration.
• paperweight-userdev automatically detects shadow and will use shadowJar as input for reobfJar. This means no extra configuration is required to use paperweight-userdev with shadow. See the shadow branch on this repository for an example usage of shadow with paperweight-userdev.
• The run-paper Gradle plugin is optional, it integrates with paperweight and allows for launching a test server with your plugin through the runServer and runMojangMappedServer tasks.
• Due to a gradle bug, independently applying paperweight-userdev to multiple projects in a build can result in errors. To work around this, apply paperweight-userdev to the root project with apply false (i.e., id("...") version "..." apply false in Kotlin DSL), and then when applying paperweight-userdev to subprojects don't include a version specification. A more advanced solution would involve adding paperweight-userdev as a dependency to your build logic, see reflection-remapper and the source-remap branch on this repo for examples of this.
• The source-remap branch on this repo has a special remapPluginSources task to remap the source code in src/main/java from spigot to Mojang mappings, outputting remapped source in /src/main/mojangMappedJava. Note that this will only remap your code, not update it from a prior version. Meaning you must be using the dev bundle for the Minecraft version your source code is for when remapping.
• paperweight-userdev doesn't provide any utilities for doing reflection. reflection-remapper is a companion library to paperweight-userdev assisting with reflection on remapped code.