Add package.json main field for TypeScript plugin

Although the `exports` field should be sufficient, some TypeScript
integrations still rely on the `main` field.

See #474 (comment)

.changeset/nervous-spiders-sip.md

@@ -0,0 +1,5 @@
+---
+'@mdx-js/typescript-plugin': patch
+---
+
+Add the main field

.changeset/silly-kiwis-think.md

@@ -0,0 +1,5 @@
+---
+'@mdx-js/typescript-plugin': patch
+---
+
+Emit type declarations

packages/typescript-plugin/package.json

@@ -13,9 +13,14 @@
   "author": "Remco Haszing <[email protected]>",
   "funding": "https://opencollective.com/unified",
   "license": "MIT",
-  "exports": "./lib/index.cjs",
+  "main": "./lib/index.cjs",
+  "exports": {
+    "types": "./types/index.d.cts",
+    "default": "./lib/index.cjs"
+  },
   "files": [
-    "lib"
+    "lib",
+    "types"
   ],
   "keywords": [
     "IntelliSense",

packages/typescript-plugin/tsconfig.json

@@ -1,9 +1,13 @@
 {
   "extends": "../../tsconfig.base.json",
+  "include": ["lib"],
   "references": [{"path": "../language-service/tsconfig.build.json"}],
   "compilerOptions": {
-    "noEmit": true,
+    "declaration": true,
+    "emitDeclarationOnly": true,
     // https://github.com/microsoft/TypeScript/issues/57652
-    "resolveJsonModule": true
+    "resolveJsonModule": true,
+    "outDir": "types",
+    "rootDir": "lib"
   }
 }