feat(tax): Add TaxRegion (#6421)

**What**
- Adds a TaxRegion entity.

**For context: High-level design of the Tax module**
- A TaxRegion scopes tax rates to a geographical place.
- You can define tax regions on two levels: country-level, province-level (this corresponds to state in US contexts)
- Each Tax Region can have a default Tax Rate.
-  [not yet done] - Each Tax Region can also have granularly defined tax rates for different products and shipping rates. For example, California can have a base rate for default products, but a reduced rate for groceries.
- Tax Rates specify if they can be combined with other rates - it's always the lowest level rate that wins.

The above allows a merchant to define their tax settings along the lines of this:

- Denmark (Region)
  - Default rate: 25% (TaxRate)
- Germany (Region)
  - Default rate: 19% (TaxRate)
  - Reduced rate (books): 9% (TaxRate w. rule)
- United States (Region)
  - Default rate: 0% (TaxRate)
  - California: (Region)
    - Default rate: 7.25% (TaxRate)
  - Arkansas: (Region)
    - Default rate: 6.5%
    - Reduced rate (groceries): 0.125% (TaxRate w. rule)

The TaxModule can then receive a list of products and the shipping address to determine what tax rates apply to the line items.

packages/tax/integration-tests/__tests__/index.ts

@@ -1,5 +1,95 @@
-describe("noop", function () {
-  it("should run", function () {
-    expect(true).toBe(true)
+import { initModules } from "medusa-test-utils"
+import { ITaxModuleService } from "@medusajs/types"
+import { Modules } from "@medusajs/modules-sdk"
+
+import { MikroOrmWrapper } from "../utils"
+import { getInitModuleConfig } from "../utils/get-init-module-config"
+
+jest.setTimeout(30000)
+
+describe("TaxModuleService", function () {
+  let service: ITaxModuleService
+  let shutdownFunc: () => Promise<void>
+
+  beforeAll(async () => {
+    const initModulesConfig = getInitModuleConfig()
+
+    const { medusaApp, shutdown } = await initModules(initModulesConfig)
+
+    service = medusaApp.modules[Modules.TAX]
+
+    shutdownFunc = shutdown
+  })
+
+  afterAll(async () => {
+    await shutdownFunc()
+  })
+
+  beforeEach(async () => {
+    await MikroOrmWrapper.setupDatabase()
+  })
+
+  afterEach(async () => {
+    await MikroOrmWrapper.clearDatabase()
+  })
+
+  it("should create a tax region", async () => {
+    const [region] = await service.createTaxRegions([
+      {
+        country_code: "US",
+        default_tax_rate: {
+          name: "Test Rate",
+          rate: 0.2,
+        },
+      },
+    ])
+
+    const [provinceRegion] = await service.createTaxRegions([
+      {
+        country_code: "US",
+        province_code: "CA",
+        parent_id: region.id,
+        default_tax_rate: {
+          name: "CA Rate",
+          rate: 8.25,
+        },
+      },
+    ])
+
+    const listedRegions = await service.listTaxRegions()
+    expect(listedRegions).toEqual(
+      expect.arrayContaining([
+        expect.objectContaining({
+          id: region.id,
+          country_code: "US",
+          province_code: null,
+          parent_id: null,
+        }),
+        expect.objectContaining({
+          id: provinceRegion.id,
+          country_code: "US",
+          province_code: "CA",
+          parent_id: region.id,
+        }),
+      ])
+    )
+
+    const rates = await service.list()
+    expect(rates).toEqual(
+      expect.arrayContaining([
+        expect.objectContaining({
+          tax_region_id: region.id,
+          rate: 0.2,
+          name: "Test Rate",
+          is_default: true,
+        }),
+        expect.objectContaining({
+          tax_region_id: provinceRegion.id,
+          rate: 8.25,
+          name: "CA Rate",
+          is_default: true,
+        }),
+      ])
+    )
   })
 })