Added a watch loop to continuously build whilst editing code. (#119)

Added "watch" command to package.json, that will watch for Elm file
changes and build on change.

Useful for keeping the cli compiler up to date quickly as code changes.

Author: rupertlssmith

package.json

@@ -14,6 +14,7 @@
     "build:node": "./scripts/build.sh node",
     "build:browser": "./scripts/build.sh browser",
     "build:bin": "./scripts/build.sh bin",
+    "watch": "onchange \"src/**/*.elm\" -- npm run build:bin",
     "test": "npm-run-all --sequential test:*",
     "test:eslint": "eslint",
     "test:elm-format-validate": "elm-format . --validate",
@@ -44,6 +45,7 @@
     "guida": "^0.3.0-alpha",
     "jest": "^29.7.0",
     "npm-run-all": "^4.1.5",
+    "onchange": "^7.1.0",
     "uglify-js": "^3.19.3"
   }
 }