[JS IR] Add to IR keep possibility similar to legacy-dce one

It helps to:

- keep declarations even if they are not reachable and not exported
- not minify names of not exported declarations

Compiler argument: -Xir-keep=A,B

Can be used for top-level declarations or for member

^KT-54118 fixed

js/js.translator/testData/box/keep/keepNestedClass.kt

@@ -0,0 +1,44 @@
+// TARGET_BACKEND: JS_IR
+// ONLY_IR_DCE
+// RUN_PLAIN_BOX_FUNCTION
+// INFER_MAIN_MODULE
+// KEEP: A.B
+
+// MODULE: keep_nested_class
+// FILE: lib.kt
+
+class A {
+    fun foo(): String {
+        return "foo"
+    }
+
+    fun bar(): String {
+        return "bar"
+    }
+
+    class B {
+        fun baz() = "baz"
+    }
+}
+
+@JsExport
+fun barA(): A {
+    return A()
+}
+
+@JsExport
+fun bar(): A.B {
+    return A.B()
+}
+
+// FILE: test.js
+function box() {
+    var a = this["keep_nested_class"].barA()
+    var b = this["keep_nested_class"].bar()
+
+    if (typeof a.foo_26di_k$ !== "undefined") return "fail 1"
+    if (typeof a.bar_232r_k$ !== "undefined") return "fail 2"
+    if (b.baz_232z_k$() != "baz") return "fail 3"
+
+    return "OK"
+}