Create rule S126: "if ... else if" constructs should end with "else" clauses (#4747)

* Add rust to rule S126

* Update RSPEC

---------

Co-authored-by: yassin-kammoun-sonarsource <yassin-kammoun-sonarsource@users.noreply.github.com>
Co-authored-by: yassin-kammoun-sonarsource <yassin.kammoun@sonarsource.com>

rules/S126/rust/metadata.json

@@ -0,0 +1,8 @@
+{
+  "tags": [
+    "clippy"
+  ],
+  "defaultQualityProfiles": [
+    "Sonar way"
+  ]
+}

rules/S126/rust/rule.adoc

@@ -0,0 +1,33 @@
+== Why is this an issue?
+
+include::../description.adoc[]
+
+=== Noncompliant code example
+
+[source,rust,diff-id=1,diff-type=noncompliant]
+----
+if x == 0 {
+    do_something();
+} else if x == 1 {
+    do_something_else();
+}
+----
+
+=== Compliant solution
+
+[source,rust,diff-id=1,diff-type=compliant]
+----
+if x == 0 {
+    do_something();
+} else if x == 1 {
+    do_something_else();
+} else {
+    panic!("Unexpected value for x");
+}
+----
+
+== Resources
+
+=== Documentation
+
+* Clippy Lints - https://rust-lang.github.io/rust-clippy/master/index.html#else_if_without_else