Change property length with operation - 1 to property lastIndex

### What's done:
- Added new rule to replace ".length - 1" to ".lastIndex"
- Added warn tests
- Added fix tests
- Updated Readme (rule 6.2.4)

(#1140)

diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter6/UseLastIndexFixTest.kt

@@ -1,6 +1,5 @@
 package org.cqfn.diktat.ruleset.chapter6
 
-
 import org.cqfn.diktat.ruleset.rules.chapter6.UseLastIndex
 import org.cqfn.diktat.util.FixTestBase
 
@@ -16,5 +15,4 @@ class UseLastIndexFixTest : FixTestBase("test/chapter6/lastIndex_change", ::UseL
     fun `fix example with incorrect use length`() {
         fixAndCompare("IncorrectUseLengthMinusOneExpected.kt", "IncorrectUseLengthMinusOneTest.kt")
     }
-
 }