Implement export of item icons and locale

Add mod and command for exporting the Factorio locale including mods,
and item icons assemled into a sprite sheet including mods.

test/integration/export.js

@@ -0,0 +1,33 @@
+const assert = require("assert").strict;
+const path = require("path");
+
+const factorio = require("lib/factorio");
+
+
+describe("Integration of lib/factorio/export", function() {
+	describe("exportLocale()", function() {
+		it("returns a nested Map with base game locale information", async function() {
+			let writePath = path.join("temp", "test", "server");
+			let testServer = new factorio.FactorioServer("factorio", writePath, {});
+			await testServer.init();
+
+			let locale = await factorio._exportLocale(testServer, new Map(), ["base"], "en");
+
+			assert(locale instanceof Map, "locale is not a map");
+			assert.equal(locale.get("entity-name.fish"), "Fish");
+			assert.equal(locale.get("entity-name.small-lamp"), "Lamp");
+			let empty_keys = 0;
+			for (let [key, value] of locale) {
+				// key is always a string
+				assert.equal(typeof key, "string");
+
+				// Value is always a string, nearly always truthy
+				if (!value) {
+					empty_keys++;
+				}
+				assert.equal(typeof value, "string");
+			}
+			assert(empty_keys < 10, `Got unusually high amount (${empty_keys}) of empty keys`);
+		});
+	});
+});