Update output target to es2018 for Node 10

As prompted by eduter on Discord, Node 10 (current game runtime)
supports es2018. This is also confirmed by this stackoverflow post[1]
and node.green [2].

This also sets the lib to "es2018" vs "esnext". With esnext it's
possible to reference standard library functions that don't exist in
es2018 and are not polyfilled by tsc. Ex. String.prototype.matchAll from
es2020. This would cause a runtime error.

If users of this library want to provide their own polyfills, they can
change the lib back to "esnext" or some other future version.

[1] https://stackoverflow.com/questions/51716406/typescript-tsconfig-settings-for-node-js-10
[2] https://node.green/#ES2018

tsconfig.json

@@ -1,8 +1,8 @@
 {
   "compilerOptions": {
     "module": "esnext",
-    "lib": ["esnext"],
+    "lib": ["es2018"],
-    "target": "es2017",
+    "target": "es2018",
     "moduleResolution": "Node",
     "outDir": "dist",
     "baseUrl": "src/",