🐛 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");

src/index.ts

@@ -1,2 +1,2 @@
-export { match, P } from "./match.js";
+export { match, P } from "./match.ts";
-export type { Matcher, Pattern } from "./pattern.js";
+export type { Matcher, Pattern } from "./pattern.ts";

src/match.ts

@@ -1,4 +1,4 @@
-import { P, type Matcher, type Pattern } from "./pattern.js";
+import { P, type Matcher, type Pattern } from "./pattern.ts";
 
 type Cases<R> = ReadonlyArray<
     readonly [Matcher<unknown>, (value: unknown) => R]

tsconfig.build.json

@@ -2,6 +2,7 @@
     "extends": "./tsconfig.json",
     "compilerOptions": {
         "noEmit": false,
+        "rewriteRelativeImportExtensions": true,
         "rootDir": "src",
         "outDir": "dist"
     },

tsconfig.json

@@ -23,6 +23,7 @@
         "noUnusedParameters": true,
         "isolatedModules": true,
         "verbatimModuleSyntax": true,
+        "allowImportingTsExtensions": true,
         "declaration": true,
         "declarationMap": true,
         "sourceMap": true,