Add .js extension in the import statement of es modules.

[pissang]

To allow us importing ES modules in the lib folder on the browsers.

Example:

<script type="importmap">
{
    "imports": {
        "zrender/": "https://unpkg.com/zrender@latest/",
        "tslib": "https://unpkg.com/tslib@2.3.1/tslib.es6.js"
    }
}
</script>
<script type="module">
if (typeof process === 'undefined' || !process.env) {
    window.process = {
        env: {
            NODE_ENV: 'development'
        }
    };
}
import PathProxy from 'zrender/lib/core/PathProxy.js'
</script>