extract "core" package, small improvements (#137)

This PR mainly refactors the code by extracting a "core" package which
is then used by the "backend" and "frontend" apps.

Apart from this the PR also contains several smaller changes like fixing
dependabot, upgrading dependencies, aligned "package.json" files, added
tests, ...

addresses step 1 of #32 
closes #136

---------

Co-authored-by: Marco Pasqualetti <marco.pasqualetti@live.com>

docs/advanced_documentation.md

@@ -76,7 +76,7 @@ GitHub Stats Extended comes with several built-in themes (e.g. `dark`, `radical`
 
 <img src="https://res.cloudinary.com/anuraghazra/image/upload/v1595174536/grs-themes_l4ynja.png" alt="GitHub Stats Extended Themes" width="600px"/>
 
-You can look at a preview for [all available themes](../backend/themes/README.md) or checkout the [theme config file](../backend/themes/index.js). Please note that we paused the addition of new themes to decrease maintenance efforts; all pull requests related to new themes will be closed.
+You can look at a preview for [all available themes](../packages/core/src/themes/README.md) or checkout the [theme config file](../packages/core/src/themes/index.js). Please note that we paused the addition of new themes to decrease maintenance efforts; all pull requests related to new themes will be closed.
 
 #### Responsive Card Theme
 
@@ -102,7 +102,7 @@ We have included a `transparent` theme that has a transparent background. This t
 
 ##### Add transparent alpha channel to a themes bg\_color
 
-You can use the `bg_color` parameter to make any of [the available themes](../backend/themes/README.md) transparent. This is done by setting the `bg_color` to a color with a transparent alpha channel (i.e. `bg_color=00000000`):
+You can use the `bg_color` parameter to make any of [the available themes](../packages/core/src/themes/README.md) transparent. This is done by setting the `bg_color` to a color with a transparent alpha channel (i.e. `bg_color=00000000`):
 
 ```md
 ![Anurag's GitHub stats](https://github-stats-extended.vercel.app/api?username=anuraghazra&show_icons=true&bg_color=00000000)
@@ -181,7 +181,7 @@ You can customize the appearance of all your cards however you wish with URL par
 | `border_color` | Card's border color. Does not apply when `hide_border` is enabled. | string (hex color) | `e4e2e2` |
 | `bg_color` | Card's background color. | string (hex color or a gradient in the form of *angle,start,end*) | `fffefe` |
 | `hide_border` | Hides the card's border. | boolean | `false` |
-| `theme` | Name of the theme, choose from [all available themes](../backend/themes/README.md). | enum | `default` |
+| `theme` | Name of the theme, choose from [all available themes](../packages/core/src/themes/README.md). | enum | `default` |
 | `cache_seconds` | Sets the cache header manually (min: 21600, max: 86400). | integer | `21600` |
 | `locale` | Sets the language in the card, you can check full list of available locales [here](#available-locales). | enum | `en` |
 | `border_radius` | Corner rounding on the card. | number | `4.5` |