fix: migrate segment subscribers + typescript support (#5904)

Fixes #6068.

**What**

- Uses new Subscriber API.
- Adds support for typescript files in Segment plugin.
- Adds a peerDependency on @medusajs/medusa for the version that introduced new Subscriber API.

packages/medusa-plugin-segment/tsconfig.admin.json

@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "module": "esnext"
+  },
+  "include": ["src/admin"],
+  "exclude": ["**/*.spec.js"]
+}