[Cleanup] Remove TS migration lines

- Removes allowJs: this repo has been full TypeScript for some time.
- Removes downlevel iteration: target is ES2022, so an ES2015-era migration workaround is probably not required.

Author: webfiltered

tsconfig.json

@@ -16,9 +16,7 @@
     "noUnusedLocals": true,
     "noUnusedParameters": true,
     "noFallthroughCasesInSwitch": true,
-    "downlevelIteration": true,
     "noImplicitOverride": true,
-    "allowJs": true,
     "baseUrl": ".",
     "paths": {
       "@/*": ["src/*"]