Improve Pan language support (#3691)

* Add a larger set of sample files for Pan

This is a fairly good cross section of Pan based on code from:
* https://github.com/quattor/template-library-examples
* https://github.com/quattor/template-library-core

* Add Pan language grammar

samples/Pan/nodes_properties.pan

@@ -0,0 +1,18 @@
+template config/nodes_properties;
+
+variable SITES ?= list('example');
+
+#variable NEW_NODES_PROPS ?= {
+
+variable NODES_PROPS = {
+    nodes_add = dict();
+    nodes_props = dict();
+    allsites = SITES;
+    ok = first(allsites, k, v);
+    while (ok) {
+        nodes_add = merge(create(format("config/%s_nodes_properties", v)), nodes_props);
+        nodes_props = merge(nodes_add[v], nodes_props);
+        ok = next(allsites, k, v);
+    };
+    nodes_props;
+};