[IMP] misc: export the validate function

The props validation code is actually quite difficult to get right.
Also, it is sometimes useful to be able to validate an object against a
specified schema. Therefore, this commit exports the standalone validate
function as an utility function.

doc/reference/props.md

@@ -243,6 +243,8 @@ class ComponentB extends owl.Component {
   };
 ```
 
+Note: the props validation code is done by using the [validate utility function](utils.md#validate).
+
 ## Good Practices
 
 A `props` object is a collection of values that come from the parent. As such,