refactor: replace gulp by webpack and npm scripts (#258)

BREAKING CHANGE: We have replaced `gulp` with `webpack` and `npm scripts` to build this theme. If you build it on your own or use build commands during the deployment, you may have to adjust your setup.

BREAKING CHANGE: The `GeekblogIcons` font is using the icon name as Unicode now. As a consequence, you have to replace all references to Icons from this font if you have customized the theme.

BREAKING CHANGE: We have refactored the search integration to split Hugo templates from JavaScript code. To get it working again, you need to adjust the `outputFormats` and `outputs` in your Hugo configuration file, as [documented](https://geekdocs.de/usage/configuration/#site-configuration).

layouts/_default/_markup/render-image.html

@@ -1,2 +1,6 @@
-<img src="{{ .Destination | safeURL }}" alt="{{ .Text }}" {{ with .Title}} title="{{ . }}"{{ end }} />
+<img
+  src="{{ .Destination | safeURL }}"
+  alt="{{ .Text }}"
+  {{ with .Title }}title="{{ . }}"{{ end }}
+/>
 {{- /* Drop trailing newlines */ -}}