build: the ES module build gets its own declarations

An ES module TypeScript consumer was given the CommonJS declarations in dist/types. With those,
`import alex2node from "alex2node"` compiled (module Node16) and then failed when Node loaded it: "The requested
module 'alex2node' does not provide an export named 'default'". The ES module build has named exports only.

tsconfig.esm.json now emits declarations next to dist/esm/*.js, under that directory's {"type": "module"}, and
"exports" selects per condition: import -> dist/esm/index.d.ts, require -> dist/types/index.d.ts. The default
import is now refused with TS1192; named imports are unchanged. "main", "module" and "types" are as before.

test/fixtures/types.mts holds the default import under @ts-expect-error, and a new test requires "types" before
"default" and a declaration for every built file. dist/: 25 files, 132,291 B -> 33 files, 155,647 B; the npm
tarball 25,125 B -> 25,837 B. npm test: 25 pass in 5.6 s.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tsconfig.esm.json

@@ -3,6 +3,7 @@
   "compilerOptions": {
     "module": "ES2020",
     "noEmit": false,
-    "outDir": "./dist/esm"
+    "outDir": "./dist/esm",
+    "declaration": true
   }
 }