Add .prettierrc.json and refactor test files for improved formatting consistency:

- Introduced `.prettierrc.json` to enforce consistent code formatting across the project.
- Updated unit and e2e test files to address formatting issues, improve readability, and ensure alignment with the new Prettier configuration.

tests/unit/invoicing-billing-period-calculations.spec.js

@@ -38,7 +38,12 @@ describe("invoicing billing period calculations", () => {
 
   it("falls back to transaction total when fixed pricing is not included", () => {
     const value = getCustomerNetAmount(
-      { transactions: [{ amount: 100, excluded: false }, { amount: 35.2, excluded: false }] },
+      {
+        transactions: [
+          { amount: 100, excluded: false },
+          { amount: 35.2, excluded: false },
+        ],
+      },
       { includeFixedPricing: false, includeTransactions: true, includeVehicleSubscriptions: false },
       { meta: { fixed_pricing: { price: 999 } } }
     );
@@ -73,4 +78,3 @@ describe("invoicing billing period calculations", () => {
     expect(totals.notBooked).toBe(170);
   });
 });
-