WIP | Add new rules to Core ruleset based on Make post

Related:
* `Universal.Namespaces.OneDeclarationPerFile` - upstream Extra WordPress#6
* `Universal.Namespaces.DisallowCurlyBraceSyntax` - upstream Extra WordPress#4
* `Universal.Namespaces.DisallowDeclarationWithoutName` - upstream Extra WordPress#50

* `Universal.UseStatements.NoLeadingBackslash` - upstream Extra WordPress#46
* `Universal.UseStatements.LowercaseFunctionConst` - upstream Extra WordPress#58

* `Universal.Constants.LowercaseClassResolutionKeyword` - upstream Extra WordPress#72

* `Generic.WhiteSpace.IncrementDecrementSpacing` - issue WordPress#1511, upstream PHPCS WordPress#2172, WordPress#2174
* `Universal.Operators.DisallowStandalonePostIncrementDecrement` - upstream Extra WordPress#65

* `Universal.Constants.UppercaseMagicConstants` - upstream Extra WordPress#64

WordPress-Core/ruleset.xml

@@ -637,6 +637,32 @@
 	<rule ref="Generic.PHP.BacktickOperator"/>
 
 
+	<!--
+	HANDBOOK NEEDS ADJUSTING
+	-->
+	<!-- Covers rule: There should be only one namespace declaration per file. -->
+	<rule ref="Universal.Namespaces.OneDeclarationPerFile"/>
+
+	<!-- Covers rules:
+		 Namespace declaration using the curly brace block syntax are not allowed.
+		 Namespace declarations without name (= explicit global namespace) are not allowed.
+		 (as those can only be created using the curly brace block syntax).
+	-->
+	<rule ref="Universal.Namespaces.DisallowCurlyBraceSyntax"/>
+
+	<!-- Covers rule: Namespace declarations without name (= explicit global namespace) are not allowed. -->
+	<rule ref="Universal.Namespaces.DisallowDeclarationWithoutName"/>
+
+
+	<!-- Covers rule: Names in an import use statement should not start with a leading backslash. -->
+	<rule ref="Universal.UseStatements.NoLeadingBackslash"/>
+
+	<!-- Covers rule: The use, function, const and as keywords should be lowercase.
+		 This is partially already covered via Generic.PHP.LowerCaseKeyword
+	-->
+	<rule ref="Universal.UseStatements.LowercaseFunctionConst"/>
+
+
 	<!--
 	#############################################################################
 	Not in the handbook: Generic sniffs.