feat: Add support namedExports

[dungjk]

Greetings,

I've experienced with esbuild-sass-plugin and esbuild for quite long time. I also work with rollup and rollup-plugin-postcss. My most wanted feature on esbuild-sass-plugin is namedExports.

What is namedExports?

// Default import
import styles from './style.scss';

const Component = () => {
    return <div className={clsx(styles.class_foo, style.class_bar)} />
}

//Named import
import { class_foo, class_bar } from './style.scss';

const Component = () => {
    return <div className={clsx(class_foo, class_bar)} />
}

Why namedExport?

• Easier to switch to esbuild from rollup.
• More convenience on importing class names from css/scss module: import { class_foo, class_bar } from 'my-scss-module.scss';
• Save some bytes by allow tree shaking remove unused import names.
• Allow cross build using esbuild + esbuild-sass-plugin on development env, rollup + rollup-plugin-postcss for production build.
• more...

Is this PR introduce breaking changes?

No, this PR open support for named import in addition to default import.
You can still use default import as before together with named import.

How to use namedExports?

See readme.md file.
I've added some document to getting started.

I've also add a new fixture to project test/fixture/named-exports and new e2e unit test..

Hope this help.

Regards.

[dungjk]

Hello @glromeo
I've completed my implement and code adjustments. Ready for reviewing.
Hope you have some time to review this PR.
Thanks and no worry, just want to make contact.

PS: This is my first contribute to this project, it take me some time to recognize your coding style 🔥 and take several changes to match with your.

[glromeo]

Thank you for this contribution... I see you put a lot of thoughts into it which I appreciate a lot!

[glromeo]

...it's the first time I see someone's else code without the urge to go and refactor it to my taste 😆
take it as a huge compliment for that matter!