Update Gradle build configuration to use best practices

[aSemy]

intellij-rust uses Gradle to build the project. However, it uses several non-idiomatic practices.

Benefits

Updating the project to use idiomatic practices would result in

• faster build speeds
• avoid work during project-configuration, so that the project will load and run faster
• easier to update to newer versions of Gradle (specifically, prepare for project isolation and configuration cache)
• simplified build scripts (move shared configuration out of the root build.gradle.kts and into convention plugins)
• Kotlin DSL accessors (which are currently not generated)
• allow for project restructuring (so subprojects could be moved and organised into a more logical structure)

Tasks

I'd like to implement the following:

• Move all of the existing build configuration properties into a specific IjRustBuildProperties class, so that they are visible in one place
• Convert the allprojects {} configuration into a convention plugin
• move subproject-specific configuration into the build.gradle.kts project of each subproject

Examples

To demonstrate the end result, please take a look at these projects which I performed similar build-logic refactoring:

• https://github.com/mockk/mockk/
• https://github.com/Kotlin/dokka/
• https://github.com/Quillraven/Fleks
• https://github.com/rjaros/kvision