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 <remcohaszing@gmail.com>",
  "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"
  }
 }