[JS] Extract sourcemap generating related files to a separate module

It's required to reuse the same infrastructure in the new backend.

js/js.dce/build.gradle.kts

@@ -8,6 +8,7 @@ dependencies {
     compile(project(":compiler:util"))
     compile(project(":js:js.ast"))
     compile(project(":js:js.translator"))
+    compileOnly(project(":js:js.sourcemap"))
     compileOnly(intellijCoreDep()) { includeJars("intellij-core") }
     compileOnly(intellijDep()) { includeJars("guava", rootProject = rootProject) }
 }