feat: conditionally export ECMAScript modules (#3377)

* feat: conditionally export ECMAScript modules

BREAKING CHANGE: With module loaders that support conditional exports
and ECMAScript modules, `import katex from 'katex';` will import the
ECMAScript module.

You can now use:
|Before                                    |After                             |
|------------------------------------------|----------------------------------|
|`require('katex/dist/contrib/[name].js')` | `require('katex/contrib/[name]')`|
|`import katex from 'katex/dist/katex.mjs'`| `import katex from 'katex'`      |
|`import 'katex/dist/contrib/[name].mjs'`  | `import 'katex/contrib/[name]'`  |

package.json

@@ -3,6 +3,33 @@
   "version": "0.13.24",
   "description": "Fast math typesetting for the web.",
   "main": "dist/katex.js",
+  "exports": {
+    ".": {
+      "require": "./dist/katex.js",
+      "import": "./dist/katex.mjs"
+    },
+    "./contrib/auto-render": {
+      "require": "./dist/contrib/auto-render.js",
+      "import": "./dist/contrib/auto-render.mjs"
+    },
+    "./contrib/mhchem": {
+      "require": "./dist/contrib/mhchem.js",
+      "import": "./dist/contrib/mhchem.mjs"
+    },
+    "./contrib/copy-tex": {
+      "require": "./dist/contrib/copy-tex.js",
+      "import": "./dist/contrib/copy-tex.mjs"
+    },
+    "./contrib/mathtex-script-type": {
+      "require": "./dist/contrib/mathtex-script-type.js",
+      "import": "./dist/contrib/mathtex-script-type.mjs"
+    },
+    "./contrib/render-a11y-string": {
+      "require": "./dist/contrib/render-a11y-string.js",
+      "import": "./dist/contrib/render-a11y-string.mjs"
+    },
+    "./*": "./*"
+  },
   "homepage": "https://katex.org",
   "repository": {
     "type": "git",