Explicitly add buffer to workspace

### Describe your request

I like the distinction between real and unreal buffers in doom, it cuts the mustard most of the time. But there are cases in which it really gets in my way (see below). In persp-mode one can add a buffer to the perspective. I would like to do the same for a workspace, that is to keep the default criterion but extended with buffers that I explicitly designated as part of the workspace. Indeed, I'm not sure whether the real/unreal filter should be applied when listing buffers inside a workspace, I believe it's already applied in advance by the hook that potentially adds a buffer to the workspace. So my proposal could be as simple as:
* Show all buffers in perspective if inside a workspace (without filtering unreal ones, that AFAICS are not added to the workspace to start with, anyway).
* Provide a convenient binding to persp-add-buffer/perp-remove-buffer that acts as a toggle.

### Briefly explain its use-case

A typical workflow of mine (and I believe it's not too peculiar) is:

1. Open some code file, be it python or org-babel.
2. Open an interpreter.
3. Evaluate in the interpreter snippets that reside in the code file that I'm editing.

I find myself wanting to quickly switch (space-`) between the code and the interpreter many times, but the interpreter of all buffers is not considered part of my workspace, because it's unreal, when actually it's one of the most frequently visited buffers. Sometimes I have the code and the interpreter side by side in two different windows, but that's not always.

Something similar happens with my scratch window, although I believe this is a more personal use case. The point stands anyway that the current workflow is somewhat rigid and could be adapted to each user needs by simply allowing the user to extend the working set, as persp-mode does. I like opinionated, but IMO this is a bit too much.