Create rule S6660: If statements should not be the only statement in else blocks (#2383)

https://github.com/SonarSource/SonarJS/issues/3916

rules/S6660/javascript/metadata.json

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+{
+  "title": "If statements should not be the only statement in else blocks",
+  "type": "CODE_SMELL",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-6660",
+  "sqKey": "S6660",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "covered"
+}

rules/S6660/javascript/rule.adoc

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+== Why is this an issue?
+
+When `if` is the only statement in the `else` block, it is better to use `else if` because it simplifies the code and makes it more readable.
+
+When using nested `if` statements, it can be difficult to keep track of the logic and understand the flow of the code. Using `else if` makes the code more concise and easier to follow.
+
+[source,javascript,diff-id=1,diff-type=noncompliant]
+----
+if (condition1) {
+    // ...
+} else {
+    if (condition2) {  // Noncompliant: 'if' statement is the only statement in the 'else' block
+        // ...
+    }
+}
+
+
+if (condition3) {
+    // ...
+} else {
+    if (condition4) { // Noncompliant: 'if' statement is the only statement in the 'else' block
+        // ...
+    } else {
+        // ...
+    }
+}
+----
+
+Fix your code by using `else if` if the nested `if` is the only statement in the `else` block.
+
+[source,javascript,diff-id=1,diff-type=compliant]
+----
+if (condition1) {
+    // ...
+} else if (condition2) {
+    // ...
+}
+
+
+if (condition3) {
+    // ...
+} else if (condition4) {
+    // ...
+} else {
+    // ...
+}
+----
+
+
+== Resources
+=== Documentation
+* link:++https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/if...else++[MDN - ``++if...else++``]

rules/S6660/metadata.json

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+{
+}