chore(!): Upgrade Node to LTS for --import support

So that we can use tsx with official --import rather than requiring (old-style) which doesn't work with string-sameness esm for some reason

mochajs/mocha#5002
mochajs/mocha-examples#76

.mocharc.json

@@ -1,4 +1,5 @@
 {
   "reporter": "dot",
-  "require": ["tsx"]
+  "extension": "ts",
+  "import": "tsx/esm"
 }

.nvmrc

@@ -1 +1 @@
-v18.16.0
+v18.19.1

package.json

@@ -13,7 +13,7 @@
     "typedoc": "typedoc",
     "circular": "madge --circular --extensions ts src/index.ts",
     "test": "npm run -s test:backend",
-    "test:backend": "mocha --extension ts --reporter spec  --recursive src/backend/tests/**/*.test.ts",
+    "test:backend": "mocha --reporter spec --recursive src/backend/tests/**/*.test.ts",
     "fileEndings": "jscodeshift --transformFrom js --transformTo none --importTypes relative --extensions=ts --parser tsx --transform codeshift/transform.ts src/backend",
     "dev": "APP_VERSION=$npm_package_version nodemon -w src/backend -x tsx src/backend/index.ts",
     "start": "APP_VERSION=$npm_package_version NODE_ENV=production tsx src/backend/index.ts",