[TASK] Exclude development-only files from dist archives

This commit adds a `.gitattributes` file which contains rules to
exclude several development-only files and folders from being
packaged into dist archives. This way, those files will not be
existent when e.g. requiring the library with Composer's default
installation method (`"preferred-install": "dist"`).

.gitattributes

@@ -0,0 +1,8 @@
+/.build export-ignore
+/.ddev export-ignore
+/.github export-ignore
+/tests export-ignore
+/.editorconfig export-ignore
+/.gitattributes export-ignore
+/.gitignore export-ignore
+/composer.lock export-ignore