package: add `yarn.lock` file

[mvidalgarcia]

partially addresses #109

Apparently, craco-less@1.18.0, released 4h ago is causing the troubles. Nothing is reported in their GH issues so far: https://github.com/DocSpring/craco-less/issues

To pin craco-less version in @semantic-ui-react/craco-less and generate a working yarn.lock, this patch was used:

diff --git a/reana-ui/package.json b/reana-ui/package.json
index 47a1b1a..5d2ee45 100644
--- a/reana-ui/package.json
+++ b/reana-ui/package.json
@@ -53,5 +53,8 @@
     "prettier": "^2.1.2",
     "semantic-ui-less": "^2.4.1",
     "source-map-explorer": "^2.5.0"
+  },
+  "resolutions": {
+    "@semantic-ui-react/craco-less/craco-less": "1.17.1"
   }
 }