add platforms option

platform stuff is still mostly special cased but

index.mjs

@@ -5,12 +5,17 @@ import {resolve, basename, extname, dirname, relative, join} from 'node:path';
 
 export function buildConfig ({
 	manifest: manifestPathRelative,
+	platforms = [],
 	scriptPlugins = [],
 	sourcemap,
 }) {
 	const manifestPath = resolve(process.cwd(), manifestPathRelative);
 	const manifestDirname = dirname(manifestPath);
 
+	if (!platforms.length) {
+		throw new Error('No platforms specified');
+	}
+
 	// Load the manifest
 	let manifestContent;
 	try {
@@ -110,8 +115,7 @@ export function buildConfig ({
 
 	const {scripts, styles, assets} = getEntryPointsFromManifest(manifestContent);
 
-	const platform = 'firefox';
-	return [
+	return platforms.flatMap(platform => [
 		...scripts.map(entrypointPath => ({
 			input: relative(process.cwd(), entrypointPath),
 			output: {
@@ -165,5 +169,5 @@ export function buildConfig ({
 				},
 			],
 		},
-	];
+	]);
 }