Update twenty-front commands (#4667)

# This PR

- Moves dev and ci scripts to the `project.json` file in the
twenty-front package
- Adds a project.json file in the root of the project with the main
start command that start both twenty-server and twenty-front
applications concurrently
- Updates the script command of the root project with the start:prod
command (replacing the start command which will be used in dev with the
help of nx)
- Add a start:prod command in the twenty-front app, replacing the start
command (now used for dev purpose)

Issue ref #4645 

@charlesBochet @FelixMalfait please let me know how can I improve it

---------

Co-authored-by: Thaïs Guigon <guigon.thais@gmail.com>

project.json

@@ -0,0 +1,12 @@
+{
+  "name": "twenty",
+  "$schema": "node_modules/nx/schemas/project-schema.json",
+  "targets": {
+    "start": {
+      "executor": "nx:run-script",
+      "options": {
+        "command": "nx run-many -t start -p twenty-server twenty-front"
+      }
+    }
+  }
+}