🐛 Use .ts extensions in imports for node --strip-types

Node's --strip-types does not rewrite import specifiers the way bundlers
and Deno do, so 'import ... from "./match.js"' literally looks for
match.js (not match.ts) and fails with ERR_MODULE_NOT_FOUND at test
time. Fix by switching source imports to .ts extensions and letting
TypeScript's rewriteRelativeImportExtensions do the rewrite at build
emit time.

Changes:
- tsconfig.json: enable allowImportingTsExtensions (works because the
  root config has noEmit: true)
- tsconfig.build.json: enable rewriteRelativeImportExtensions so the
  emitted dist/*.js files keep './match.js' style imports (correct for
  consumers), not './match.ts'
- src/index.ts: import './match.ts' and './pattern.ts'
- src/match.ts: import './pattern.ts'
- src/index.test.ts: import './index.ts'

Verified: npm run test runs tsc --noEmit (passes) and node --test
--strip-types src/ — all 6 tests pass. tsc -p tsconfig.build.json
emits dist/*.js with './match.js' imports as before; consumers see no
change.

tsconfig.json

@@ -23,6 +23,7 @@
         "noUnusedParameters": true,
         "isolatedModules": true,
         "verbatimModuleSyntax": true,
+        "allowImportingTsExtensions": true,
         "declaration": true,
         "declarationMap": true,
         "sourceMap": true,
@@ -34,4 +35,4 @@
         "types": ["node"]
     },
     "include": ["src"]
-}
+}