Implement KotlinWordsScanner

This will allow to correctly calculate the file modification crc based on it. this is needed for build.gradle.kts files which show notification about necessity of importing project. Otherwise this notification is shown on every file modification (after virtualFile save)

idea/src/org/jetbrains/kotlin/idea/findUsages/KotlinFindUsagesProvider.kt

@@ -1,17 +1,6 @@
 /*
- * Copyright 2010-2016 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
 package org.jetbrains.kotlin.idea.findUsages
@@ -35,7 +24,7 @@ class KotlinFindUsagesProvider : FindUsagesProvider {
     override fun canFindUsagesFor(psiElement: PsiElement): Boolean =
         psiElement is KtNamedDeclaration
 
-    override fun getWordsScanner(): WordsScanner? = null
+    override fun getWordsScanner(): WordsScanner? = KotlinWordsScanner()
 
     override fun getHelpId(psiElement: PsiElement): String? = null
 

idea/src/org/jetbrains/kotlin/idea/findUsages/KotlinWordsScanner.kt

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+package org.jetbrains.kotlin.idea.findUsages
+
+import com.intellij.lang.cacheBuilder.DefaultWordsScanner
+import com.intellij.psi.tree.TokenSet
+import org.jetbrains.kotlin.lexer.KotlinLexer
+import org.jetbrains.kotlin.lexer.KtTokens.*
+
+class KotlinWordsScanner : DefaultWordsScanner(
+    KotlinLexer(),
+    TokenSet.create(IDENTIFIER).also { KEYWORDS },
+    COMMENTS,
+    TokenSet.create(INTEGER_LITERAL, CHARACTER_LITERAL, FLOAT_LITERAL, NULL_KEYWORD, TRUE_KEYWORD, FALSE_KEYWORD).also { STRINGS }
+)