[FirstRun] Use generic stub file for reuse

src/firstRun/darwin.js

@@ -1,2 +1 @@
-// Stub in normal Discord
-exports.performFirstRunTasks = () => {};
+module.exports = require('./stub');

src/firstRun/index.js

@@ -1,3 +1,2 @@
 // Stub for now at least
-
 module.exports = require('./' + process.platform);

src/firstRun/linux.js

@@ -1,2 +1 @@
-// Stub in normal Discord
-exports.performFirstRunTasks = () => {};
+module.exports = require('./stub');

src/firstRun/stub.js

@@ -0,0 +1,2 @@
+// Generic exports for stub (Linux and Darwin use it), normal Discord
+exports.performFirstRunTasks = () => {};