Move tests out of src/ into a top-level tests/ directory

src/ now contains only source code; tests live alongside it under
tests/. Bun's test runner discovers '**/*.test.ts' so no test-runner
config change is needed.

Changes:
  - tests/config.test.ts        (was src/config.test.ts)
  - tests/configFile.test.ts    (was src/configFile.test.ts)
  - Imports updated: ./config.ts -> ../src/config.ts (likewise for
    configFile.ts and errors.ts).
  - tsconfig.json include adds 'tests/**/*.ts' so tsc type-checks
    the test files too.

All 25 tests still pass; tsc clean.

tsconfig.json

@@ -12,5 +12,5 @@
     "resolveJsonModule": true,
     "noUncheckedIndexedAccess": true
   },
-  "include": ["src/**/*.ts"]
+  "include": ["src/**/*.ts", "tests/**/*.ts"]
 }