add support for IFT (IDE Features Trainer)

[SingingBush]

In recent versions of IDEA there is a plugin with the id of training which we can use as an optional dependency (it'll need to be added to the plugins array in build.gradle as well)

<depends optional="true" config-file="training.xml">training</depends>

This will give some extension points that can be setup with something like:

    <extensions defaultExtensionNs="training">
        <!-- with the 'IDE Features Trainer' (id: 'training') plugin installed (included in Intellij IDEA) -->
        <ift.language.extension language="D" defaultProductName="IDEA" learnWindowAnchor="right"
                                implementationClass="io.github.intellij.dlanguage.training.DlangSupport"/>
        <ift.learning.course language="D" implementationClass="io.github.intellij.dlanguage.training.DlangLearningCourse"/>
    </extensions>

class DlangSupport : AbstractLangSupport() {

    override val primaryLanguage: String
        get() = DLanguage.id

    override fun applyToProjectAfterConfigure(): (Project) -> Unit = { newProject ->
        invokeLater { ProjectUtils.markDirectoryAsSourcesRoot(newProject, "src") }
    }

    override fun checkSdk(sdk: Sdk?, project: Project) {}

    override fun isSdkConfigured(project: Project): Boolean {
        return ProjectRootManager.getInstance(project).projectSdk?.sdkType is DlangSdkType
    }
}

this is very rough work in progress (look at the equivalent Java training)

/**
 * Add a training course for "D" language
 */
class DlangLearningCourse : LearningCourseBase(DLanguage.id) {

    override fun modules(): Collection<IftModule> = stableModules()

    private fun stableModules() = listOf(
        LearningModule(id = "D.Essential",
            name = LessonsBundle.message("essential.module.name"),
            description = LessonsBundle.message("essential.module.description", LessonUtil.productName),
            primaryLanguage = langSupport,
            moduleType = LessonType.SCRATCH) {
            listOf(
                DlangBasicCompletionLesson(),
                // todo: add more lessons
            )
        }
    )

    class DlangBasicCompletionLesson : KLesson("Basic completion", LessonsBundle.message("basic.completion.lesson.name")) {
        private val sample = parseLessonSample("""
            import std.stdio, std.array, std.algorithm;
            void main() {
                stdin.byLineCopy.
            }
            """.trimIndent())

        override val lessonContent: LessonContext.() -> Unit = {
            prepareSample(sample)
            task {
                text("Try an auto complete")
                //action("EditorChooseLookupItem")
            }
            caret(2, 21) 
            // todo: finish this code
        }
    }
}

It's worth checking the following readme as well: https://github.com/JetBrains/intellij-community/blob/9fdd68f908db0b6bb6e08dc33fafb26e2e4712af/plugins/ide-features-trainer/src/HowToCreateLessons.md