Update to webpack 4 + misc build process changes

ext/build.js

@@ -1,20 +1,29 @@
-const spawn = require('child_process').spawn;
+const { spawn } = require('child_process');
 const argv = require('minimist')(process.argv.slice(2));
 
-const appId = argv.appId || "19A6F4AE";
+
+const extensionName = "fx_cast";
+const extensionId = "fx_cast@matt.tf";
+const extensionVersion = "0.0.1";
+
+// Default argument values
+const { mirroringAppId = "19A6F4AE"
+      , mode = "development" } = argv;
 
 const child = spawn(
-    `webpack --env.appId=${appId} `
-        + '&& web-ext build '
-            + '--overwrite-dest '
-            + '--source-dir '
-            + '../dist/ext/unpacked '
-            + '--artifacts-dir '
-            + '../dist/ext '
-        + '&& mv ../dist/ext/*.zip ../dist/ext/ext.xpi'
+    `webpack --env.extensionName=${extensionName} `
+          + `--env.extensionId=${extensionId} `
+          + `--env.extensionVersion=${extensionVersion} `
+          + `--env.mirroringAppId=${mirroringAppId} `
+          + `--mode=${mode} `
+          + `${argv.watch ? "--watch" : ""} `
+        + `&& web-ext build --overwrite-dest `
+                         + `--source-dir ../dist/ext `
+                         + `--artifacts-dir ../dist/ext `
   , {
         shell: true
     }
 );
+
 child.stdout.pipe(process.stdout);
 child.stderr.pipe(process.stderr);