Add build.sh

There are no docs for building the app.asar yourself, I'm making this file for people who want to. (Just copied from the github action)

build.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+# Builds the app.asar
+
+# Prerequisites
+npm install -g asar
+
+# Clean up previous builds
+rm app.asar
+
+bash scripts/injectPolyfills.sh
+
+# Set the version
+sed -i -e "s/nightly/nightly-$(git rev-parse HEAD | cut -c 1-7)/" src/index.js
+node scripts/strip.js
+
+# Build the asar
+npx asar pack src app.asar