[TomasVotruba]

Adds a fixer that strips the redundant leading backslash from the true, false and null constants:

return \true;   // ->   return true;
return \false;  // ->   return false;
return \null;   // ->   return null;

• PHP: Symplify\CodingStandard\Fixer\ConstantNotation\RemoveSlashFromNativeConstantFixer, added to the common namespaces set.
• blink: ported as RemoveSlashFromNativeConstant (registry, priority 0, source map, README rule counts).
• Casing is preserved; namespace segments like Foo\True\Value and \true\Bar are left untouched.

Covered by PHPUnit fixture tests and a Go test.