Generate relative paths in JS source maps

Also, add CLI options to manipulate prefixes of path

See KT-4078

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/dsl/KotlinJsOptions.kt

@@ -48,6 +48,18 @@ interface KotlinJsOptions  : org.jetbrains.kotlin.gradle.dsl.KotlinCommonOptions
      */
      var sourceMap: kotlin.Boolean
 
+    /**
+     * Prefix for paths in a source map
+     * Default value: null
+     */
+     var sourceMapPrefix: kotlin.String?
+
+    /**
+     * Base directories which are used to calculate relative paths to source files in source map
+     * Default value: null
+     */
+     var sourceMapSourceRoots: kotlin.String?
+
     /**
      * Generate JS files for specific ECMA version
      * Possible values: "v5"

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/dsl/KotlinJsOptionsBase.kt

@@ -59,6 +59,16 @@ internal abstract class KotlinJsOptionsBase : org.jetbrains.kotlin.gradle.dsl.Ko
         get() = sourceMapField ?: false
         set(value) { sourceMapField = value }
 
+    private var sourceMapPrefixField: kotlin.String?? = null
+    override var sourceMapPrefix: kotlin.String?
+        get() = sourceMapPrefixField ?: null
+        set(value) { sourceMapPrefixField = value }
+
+    private var sourceMapSourceRootsField: kotlin.String?? = null
+    override var sourceMapSourceRoots: kotlin.String?
+        get() = sourceMapSourceRootsField ?: null
+        set(value) { sourceMapSourceRootsField = value }
+
     private var targetField: kotlin.String? = null
     override var target: kotlin.String
         get() = targetField ?: "v5"
@@ -81,6 +91,8 @@ internal abstract class KotlinJsOptionsBase : org.jetbrains.kotlin.gradle.dsl.Ko
         noStdlibField?.let { args.noStdlib = it }
         outputFileField?.let { args.outputFile = it }
         sourceMapField?.let { args.sourceMap = it }
+        sourceMapPrefixField?.let { args.sourceMapPrefix = it }
+        sourceMapSourceRootsField?.let { args.sourceMapSourceRoots = it }
         targetField?.let { args.target = it }
         typedArraysField?.let { args.typedArrays = it }
     }
@@ -98,6 +110,8 @@ internal fun org.jetbrains.kotlin.cli.common.arguments.K2JSCompilerArguments.fil
     noStdlib = true
     outputFile = null
     sourceMap = false
+    sourceMapPrefix = null
+    sourceMapSourceRoots = null
     target = "v5"
     typedArrays = false
 }