Add types to exports (fixes issue muratgozel#38 for ES imports)

See TypeScript issue here: microsoft/TypeScript#52363
I don't entirely follow what TS is doing, but I've based this commit on a PR
linked from that discussion: gxmari007/vite-plugin-eslint#60

I don't think this will work for CJS imports as TS seems to be pedantic around
file suffixes -- we may need to copy or wrap the .d.ts file somehow.
I'll investigate that separately.

package.json

@@ -6,21 +6,33 @@
   "exports": {
     ".": {
       "node": {
-        "import": "./dist/node/mimetext.es.js",
+        "import": {
+          "types": "./types/index.d.ts",
+          "default": "./dist/node/mimetext.es.js"
+        },
         "require": "./dist/node/mimetext.cjs",
         "default": "./dist/node/mimetext.cjs"
       },
-      "import": "./dist/browser/mimetext.es.js",
+      "import": {
+        "types": "./types/index.d.ts",
+        "default": "./dist/browser/mimetext.es.js"
+      },
       "require": "./dist/browser/mimetext.cjs",
       "default": "./dist/browser/mimetext.cjs"
     },
     "./browser": {
-      "import": "./dist/browser/mimetext.es.js",
+      "import": {
+        "types": "./types/index.d.ts",
+        "default": "./dist/browser/mimetext.es.js"
+      },
       "require": "./dist/browser/mimetext.cjs",
       "default": "./dist/browser/mimetext.cjs"
     },
     "./node": {
-      "import": "./dist/node/mimetext.es.js",
+      "import": {
+        "types": "./types/index.d.ts",
+        "default": "./dist/node/mimetext.es.js"
+      },
       "require": "./dist/node/mimetext.cjs",
       "default": "./dist/node/mimetext.cjs"
     },
@@ -97,4 +109,4 @@
   "overrides": {
     "xml2js": "0.5.0"
   }
-}
+}