chore(repo): use .mts for shared Vitest config; update alias test script/docs

- Rename .config/vitest.config.ts -> .config/vitest.config.mts per review
- Update packages/alias test script to reference .mts
- Update CONTRIBUTING to mention .mts config

Author: CharlieHelps

.config/vitest.config.ts renamed to .config/vitest.config.mts

@@ -30,7 +30,7 @@ Any code change should be submitted as a pull request. Our guidelines for Pull R
 
 Some packages use Vitest for tests. When converting or running tests locally inside a package directory (e.g. `packages/alias`):
 
-- Tests import the package under test via `~package`. A shared config at `.config/vitest.config.ts` provides a runtime alias that resolves `~package` to the current working directory.
+- Tests import the package under test via `~package`. A shared config at `.config/vitest.config.mts` provides a runtime alias that resolves `~package` to the current working directory.
 - Snapshots are written alongside each test under a local `.snapshots` directory.
 
 Example (from a package directory):

.github/CONTRIBUTING.md

@@ -32,7 +32,7 @@
     "prerelease": "pnpm build",
     "pretest": "pnpm build",
     "release": "pnpm --workspace-root package:release $(pwd)",
-    "test": "vitest --config ../../.config/vitest.config.ts run"
+    "test": "vitest --config ../../.config/vitest.config.mts run"
   },
   "files": [
     "dist",