implement sapper upgrade

src/cli/upgrade.ts

@@ -0,0 +1,53 @@
+import * as fs from 'fs';
+import chalk from 'chalk';
+
+export default async function upgrade() {
+	const upgraded = [
+
+	].filter(Boolean);
+		await upgrade_sapper_main()
+	if (upgraded.length === 0) {
+		console.log(`No changes!`);
+	}
+}
+
+async function upgrade_sapper_main() {
+	const _2xx = read('templates/2xx.html');
+	const _4xx = read('templates/4xx.html');
+	const _5xx = read('templates/5xx.html');
+
+	const pattern = /<script src='\%sapper\.main\%'><\/script>/;
+
+	let replaced = false;
+
+	['2xx', '4xx', '5xx'].forEach(code => {
+		const file = `templates/${code}.html`
+		const template = read(file);
+		if (!template) return;
+
+		if (/\%sapper\.main\%/.test(template)) {
+			if (!pattern.test(template)) {
+				console.log(chalk.red(`Could not replace %sapper.main% in ${file}`));
+			} else {
+				write(file, template.replace(pattern, `%sapper.scripts%`));
+				console.log(chalk.green(`Replaced %sapper.main% in ${file}`));
+				replaced = true;
+			}
+		}
+	});
+
+	return replaced;
+}
+
+function read(file: string) {
+	try {
+		return fs.readFileSync(file, 'utf-8');
+	} catch (err) {
+		console.error(err);
+		return null;
+	}
+}
+
+function write(file: string, data: string) {
+	fs.writeFileSync(file, data);
+}