[JS IR] Add CLI flag and configuration key about partial linkage

- make possible to turn on/off linkage if klib contains unbound symbols
 - update jsExtraHelp test data

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JSCompilerArguments.kt

@@ -235,6 +235,9 @@ class K2JSCompilerArguments : CommonCompilerArguments() {
     @Argument(value = "-Xerror-tolerance-policy", description = "Set up error tolerance policy (NONE, SEMANTIC, SYNTAX, ALL)")
     var errorTolerancePolicy: String? by NullableStringFreezableVar(null)
 
+    @Argument(value = "-Xpartial-linkage", description = "Allow unlinked symbols")
+    var partialLinkage: Boolean by FreezableVar(false)
+
     @Argument(value = "-Xwasm", description = "Use experimental WebAssembly compiler backend")
     var wasm: Boolean by FreezableVar(false)