grammer.ne: add polyfill for Array.flat (#43)

* restore node 10 testing.

Author: msimerson

.github/workflows/ci-test.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest ]
-        node-version: [12.x, 14.x]
+        node-version: [10.x, 12.x, 14.x]
       fail-fast: false
 
     steps:

grammar.ne

@@ -1,6 +1,27 @@
 main -> Mailbox | Path
 
 @{%
+// from https://github.com/jonathantneal/array-flat-polyfill
+if (!Array.prototype.flat) {
+	Object.defineProperty(Array.prototype, 'flat', {
+		configurable: true,
+		value: function flat () {
+			var depth = isNaN(arguments[0]) ? 1 : Number(arguments[0]);
+
+			return depth ? Array.prototype.reduce.call(this, function (acc, cur) {
+				if (Array.isArray(cur)) {
+					acc.push.apply(acc, flat.call(cur, depth - 1));
+				} else {
+					acc.push(cur);
+				}
+
+				return acc;
+			}, []) : Array.prototype.slice.call(this);
+		},
+		writable: true
+	});
+}
+
 function flat_string(d) {
   if (d) {
     if (Array.isArray(d))