Add set operator to make the work with StringBuilder more list-like.

Make StringBuilder.set jvm-only.
Move jvm-specific test.

libraries/stdlib/src/kotlin/text/StringBuilder.kt

@@ -40,3 +40,9 @@ public fun StringBuilder.append(vararg value: Any?): StringBuilder {
         append(item)
     return this
 }
+
+/**
+ * Sets the character at the specified [index] to the specified [value].
+ */
+@kotlin.jvm.JvmVersion
+public operator fun StringBuilder.set(index: Int, value: Char): Unit = this.setCharAt(index, value)

libraries/stdlib/test/text/StringBuilderJVMTest.kt

@@ -0,0 +1,15 @@
+package test.text
+
+import kotlin.*
+import kotlin.test.*
+import org.junit.Test as test
+
+class StringBuilderJVMTest() {
+    @test fun getAndSetChar() {
+        val sb = StringBuilder("abc")
+        sb[1] = 'z'
+
+        assertEquals("azc", sb.toString())
+        assertEquals('c', sb[2])
+    }
+}