♻️ Extract `rpgdm-core.el'

Prior to this commit, I experienced the following use case:

Given I have the following code:

```emacs-lisp
(require 'rpgdm-dice "~/git/emacs-rpgdm/rpgdm-dice.el")
(require 'rpgdm-tables "~/git/emacs-rpgdm/rpgdm-tables.el")
(require 'rpgdm-tables-dice "~/git/emacs-rpgdm/rpgdm-tables-dice.el")
(require 'rpgdm-tables-freq "~/git/emacs-rpgdm/rpgdm-tables-freq.el")

(setq rpgdm-base "~/git/emacs-rpgdm/")
```

And I call `rpgdm-tables-load`
And load all the tables.

When I then call `rpgdm-tables-choose`

Then I get the following error:

```
let*: Symbol’s function definition is void: rpgdm-message
```

With this commit I'm able to skip requiring `rpgdm` and thus only use a
segment of the `rpgdm` package ecosystem.

README.org

@@ -99,6 +99,7 @@ The second thing I realized is that Org's links can call Emacs functions. This a
 My initial ideas for listing a bunch of random NPC names and having a link that displayed one of them, got supplanted for the ideas I described above.
 * Code
 What do I have here:
+  - [[file:rpgdm-core.el][rpgdm-core]] :: Package that provides common functionality; namely in messaging and the =rpgdm-last-results= ring.
   - [[file:rpgdm.el][rpgdm]] :: Primary interface offering:
     - =rpgdm-mode=, plus a Hydra interface for easily calling the rest of these functions.
     - =rpgdm-yes-and-50/50=, flip a coin and make a give a result with or without complications or bonuses.