checking in some xsd changes

schema/lib/types/std.xsd

@@ -52,6 +52,18 @@
         </xs:restriction>
     </xs:simpleType>
 
+    <!-- https://semver.org/ -->
+    <!-- Not perfect; it's more permissible than official spec but will do nicely for basic check. -->
+    <!--
+         The XML pattern engine standard is... pretty limited but:
+         https://semver.org/#is-there-a-suggested-regular-expression-regex-to-check-a-semver-string
+    -->
+    <xs:simpleType name="t_std_semver">
+        <xs:restriction base="xs:token">
+            <xs:pattern value="v?([1-9]+([0-9])*\.){2}\.[1-9]([0-9)*(-[.0-9A-Za-z-]+)?(\+[.0-9A-Za-z-]+)?"/>
+        </xs:restriction>
+    </xs:simpleType>
+
     <xs:simpleType name="t_std_uri">
         <xs:restriction base="xs:anyURI">
             <xs:pattern value="\s*(https?|ftps?|file)://.+\s*"/>