improvement on issue 55: TZ is not recognized as a command or cmdlet …

…in Powershell environment on Windows system.

Add dev-dependency cross-env and update the 'test' script to solve this incompatibility issue.

package.json

@@ -40,14 +40,15 @@
     "rw": "1"
   },
   "devDependencies": {
+    "cross-env": "^7.0.3",
     "csv-spectrum": "1",
     "eslint": "7",
     "mocha": "8",
     "rollup": "2",
     "rollup-plugin-terser": "7"
   },
   "scripts": {
-    "test": "TZ=America/Los_Angeles mocha 'test/**/*-test.js' && eslint src test",
+    "test": "cross-env TZ=America/Los_Angeles mocha 'test/**/*-test.js' && eslint src test",
     "prepublishOnly": "rm -rf dist && yarn test && rollup -c",
     "postpublish": "git push && git push --tags && cd ../d3.github.com && git pull && cp ../${npm_package_name}/dist/${npm_package_name}.js ${npm_package_name}.v${npm_package_version%%.*}.js && cp ../${npm_package_name}/dist/${npm_package_name}.min.js ${npm_package_name}.v${npm_package_version%%.*}.min.js && git add ${npm_package_name}.v${npm_package_version%%.*}.js ${npm_package_name}.v${npm_package_version%%.*}.min.js && git commit -m \"${npm_package_name} ${npm_package_version}\" && git push && cd -"
   },