Treeshake functions properly

[mischnic]

↪️ Pull Request

This fixes an issue where functions weren't getting treeshaken if two or more function were exported.
This doesn't seem to have a big effect on larger apps.

Closes #2414, closes #2297

🔦 Context

Before treeshake() runs in the concat packager, this is the generated asset javascript:

	var $RK1r$exports = {};
	function $RK1r$export$add(r, $) {
		return r + $;
	}
	function $RK1r$export$sub(r, $) {
		return r - $;
	}
	($RK1r$exports.add = $RK1r$export$add),
		($RK1r$exports.sub = $RK1r$export$sub);

After treeshaking: (neither add nor sub are actually used!)

	function $RK1r$export$add(r, $) {
		return r + $;
	}
	function $RK1r$export$sub(r, $) {
		return r - $;
	}
	$RK1r$export$add, $RK1r$export$sub;

Notice how ($RK1r$exports.add = $RK1r$export$add), ($RK1r$exports.sub = $RK1r$export$sub); prevented the sub function from being removed.

That line was introduced by running terser in JSAsset:

// ...
$RK1r$exports.add = $RK1r$export$add;
//...
$RK1r$exports.sub = $RK1r$export$sub;

becomes:

($RK1r$exports.add = $RK1r$export$add), ($RK1r$exports.sub = $RK1r$export$sub);

That comma expression isn't getting handled properly by the treeshaker.
This is why bundling with --no-minify behaved correctly: terser didn't run and the assignments are multiple expressions.

✔️ PR Todo

• Added/updated unit tests for this change
• Included links to related issues/PRs

[devongovett]

Thanks for rebasing!