Formatter: handle spaces around 'is' keyword (#874)

Fixes #KT-12548

idea/formatter/src/org/jetbrains/kotlin/idea/formatter/kotlinSpacingRules.kt

@@ -166,6 +166,7 @@ fun createSpacingBuilder(settings: CodeStyleSettings, builderUtil: KotlinSpacing
             afterInside(IDENTIFIER, FUN).spacing(0, 0, 0, false, 0)
 
             around(AS_KEYWORD).spaces(1)
+            around(IS_KEYWORD).spaces(1)
             aroundInside(IDENTIFIER, BINARY_EXPRESSION).spaces(1)
 
             // before LPAR in constructor(): this() {}

idea/testData/formatter/Is.after.kt

@@ -0,0 +1,5 @@
+fun f() {
+    if (1 is Int) {
+
+    }
+}

idea/testData/formatter/Is.kt

@@ -0,0 +1,5 @@
+fun f() {
+    if (1   is  Int) {
+
+    }
+}

idea/tests/org/jetbrains/kotlin/formatter/FormatterTestGenerated.java

@@ -349,6 +349,12 @@ public class FormatterTestGenerated extends AbstractFormatterTest {
             doTest(fileName);
         }
 
+        @TestMetadata("Is.after.kt")
+        public void testIs() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/formatter/Is.after.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("KDoc.after.kt")
         public void testKDoc() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("idea/testData/formatter/KDoc.after.kt");