Add `async: false` setting for `+client.js`

[dajinchu]

Description

I need to run some JS before DOMContentLoaded.
My options right now are:

1. write it as plain JS and inject via Head. This works OK until the code gets complex enough / requires dependencies
2. compile it separately and add inject via Head

It would be ideal if we could set +client.js to not be async.

[brillout]

It would be ideal if we could set +client.js to not be async.

I agree that would be a nice enhancement.

Alternative feature request: being able to inject <script> tags (e.g. using +Head) as simply as injecting <script> tags with index.html when using vanilla Vite.