[#526] Add Nix Flakes configuration to handle unified developer setups

Implementing Nix Flakes is essential to combine application modules
within the project, ensuring easy access and utilization from the root
directory. By adding the Nix Flakes configuration in this commit,
developers can streamline the setup process for various modules,
enhancing project manageability and organization. This aligns with the
user story's goal of leveraging Nix Flakes for improved accessibility to
and usage of application modules.

Changes:
- Included Nix Flakes configuration to manage unified developer setups
  effectively.
- Enhanced the project structure by enabling easy access and utilization
  of application modules from the root directory.
- Streamlined the setup process for different modules, ensuring
  consistency and ease of use within the project environment.

CHANGELOG.md

@@ -36,6 +36,7 @@ changes.
 - Vitest unit tests added for utils functions [Issue 81](https://github.com/IntersectMBO/govtool/issues/81)
 - i18next library added to FE [Issue 80](https://github.com/IntersectMBO/govtool/issues/80)
 - Add possibility to vote on behalf of myself - Sole Voter [Issue 119](https://github.com/IntersectMBO/govtool/issues/119)
+- Added Nix Flakes configurtion to handle unified developers setup [Issue 526](https://github.com/IntersectMBO/govtool/issues/526)
 
 ### Fixed