build: target Node.js 20 or above (#218)

Update Node.js version target to 20 or above.

* Change the `engines` field in `package.json` to specify Node.js version ">=20".
* Update the `.nvmrc` file to specify Node.js version 20.

BREAKING CHANGE: Now requires Node.js 20 or newer, this version likely still works on older versions but is untested. If you are staying on an older version of Node.js, then you probably want to wait with this update.

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Author: Addono

.nvmrc

@@ -1,2 +1,2 @@
-16
+20
 

package.json

@@ -11,7 +11,7 @@
     "src"
   ],
   "engines": {
-    "node": ">=16"
+    "node": ">=20"
   },
   "scripts": {
     "start": "tsdx watch",