fix: Set esbuild target to es6 to be backwards compatible. (#157)

`rollup-plugin-esbuild`
[v6](https://github.com/egoist/rollup-plugin-esbuild/releases/tag/v6.0.0)
defaults to es2020 by default and no longer respects tsconfig.json's
target. This PR explicitly sets esbuild to target ES2015 aka ES6 so the
build is backwards compatible.

Fixes this
[issue](#283 (comment)).

Author: yusinto

rollup.config.ts

@@ -5,7 +5,7 @@ import json from '@rollup/plugin-json';
 import resolve from '@rollup/plugin-node-resolve';
 import terser from '@rollup/plugin-terser';
 
-const plugins = [resolve(), esbuild(), json(), terser(), filesize()];
+const plugins = [resolve(), esbuild({ target: 'es6' }), json(), terser(), filesize()];
 const external = /node_modules/;
 
 export default [