🐛 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.

src/index.test.ts

@@ -1,7 +1,7 @@
 import { test } from "node:test";
 import { strict as assert } from "node:assert";
 import { expectTypeOf } from "expect-type";
-import { match, P, type Matcher } from "./index.js";
+import { match, P, type Matcher } from "./index.ts";
 
 test("match returns a builder", () => {
     const builder = match("x");