Improve compatibility with Gradle Provider API

[aSemy]

Currently the properties that Knit considers are not compatible with Gradle lazy configuration

kotlinx-knit/src/KnitPlugin.kt

Lines 97 to 104 in c4e77da

	var siteRoot: String? = globalDefaults.siteRoot
	var moduleRoots: List<String> = globalDefaults.moduleRoots
	var moduleMarkers: List<String> = globalDefaults.moduleMarkers
	var moduleDocs: String = globalDefaults.moduleDocs
	var files: FileCollection? = null
	var rootDir: File? = null
	var dokkaMultiModuleRoot: String = globalDefaults.dokkaMultiModuleRoot
	var defaultLineSeparator: String? = null

This is a particular problem with the Knit files. I would like to be able to share the Markdown files using Gradle best practices, but because the Knit files use FileCollection, not a ConfigurableFileCollection, the files must be evaluated instantly, not on-demand.

Here is an example of how the properties could be made compatible with the Gradle Provider API

abstract class KnitPluginExtension {
    abstract val knitVersion: Property<String>
    abstract val siteRoot: Property<String>
    abstract val moduleRoots: ListProperty<String>
    abstract val moduleMarkers: ListProperty<String>
    abstract val moduleDocs: Property<String>
    abstract val files: ConfigurableFileCollection
    abstract val rootDir: RegularFileProperty
    abstract val dokkaMultiModuleRoot: Property<String>
    abstract val defaultLineSeparator: Property<String>
}