add embeddedAttrJsExtractor

[youthlin]

Hello, I want to extract message from element's attribute like this:

<template>
  <span :title="_xn('title', '%{n} apple', '%{n} apples', n, {n:''+n})">
    see my title
  </span>
</template>

I found the code from https://github.com/jshmrtn/vue3-gettext/blob/master/scripts/attributeEmbeddedJsExtractor.ts
I think it would be better implement by this repo, so that it can be relied upon and used by more repositories

[lukasgeiter]

Thanks for your PR!

In addition to the comments I've left in the code, I would really like to see some tests specifically for the new extractor. Take a look at the existing tests for embeddedJs for some inspiration.

[lukasgeiter]

Released with v3.7.0