Add support for Gradle assignment operator (#479)

Summary:
Fixes #411.

Gradle added support for a [Kotlin assignment operator](https://blog.gradle.org/simpler-kotlin-dsl-property-assignment) to simplify property assignment.

This is done via a compiler plugin and this import: `import org.gradle.kotlin.dsl.assign` - which ktfmt detects as unused and removes.

This PR adds an `assign` operator to the operators in `com.facebook.ktfmt.format.RedundantImportDetector` and a corresponding test to ensure the import remains.

Pull Request resolved: #479

Reviewed By: cortinico

Differential Revision: D58440177

Pulled By: hick209

fbshipit-source-id: 89490d8542d38fa652824569417ef7be8dc66e63

core/src/main/java/com/facebook/ktfmt/format/RedundantImportDetector.kt

@@ -76,7 +76,11 @@ internal class RedundantImportDetector(val enabled: Boolean) {
             // Property delegation operators
             "getValue",
             "setValue",
-            "provideDelegate")
+            "provideDelegate",
+            // assign operator - Gradle compiler plugin
+            // https://blog.gradle.org/simpler-kotlin-dsl-property-assignment
+            "assign",
+        )
 
     private val COMPONENT_OPERATOR_REGEX = Regex("component\\d+")
 

core/src/test/java/com/facebook/ktfmt/format/FormatterTest.kt

@@ -1451,6 +1451,7 @@ class FormatterTest {
               |import com.example.timesAssign
               |import com.example.unaryMinus
               |import com.example.unaryPlus
+              |import org.gradle.kotlin.dsl.assign
               |"""
               .trimMargin())