Apply badMutable plugin locally

While this is not technically a bug, I do believe that this is a very important feature for compatibility with `moment` based libraries.

**Current behavior**
Applying a plugin (using `.extend()`) alters the global `dayjs` constructor. For `dayjs` to be a replacement for `moment` in e.g. third party libraries, it is sometimes necessary to use the `badMutable` plugin.

But when this plugin is applied, whether manually or by some third party code, all `dayjs` instances in the code base suddenly become mutable, which can lead to unexpected behavior and nasty bugs.

**Expected behavior**
It should be possible to apply the `badMutable` plugin locally, to prevent all `dayjs` instances to suddenly become mutable.

By looking at the source code, I couldn't find a way to make this possible.

Any help or suggestions, would be highly appreciated!!

Thanks :D

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There has been a discussion on this before: #655
And this exact issue has let [`react-day-picker`](https://github.com/gpbl/react-day-picker) (~4.6K ⭐) to stop using dayjs.

Start by reviewing the plugin application path around `.extend()` and the `badMutable` plugin, then read the prior discussion in #655 for constraints and rejected approaches. The work is done when `badMutable` can be applied locally without making unrelated Day.js instances mutable, while preserving compatibility for moment-based libraries.

Written by the indexing model from the issue text.