feat(build): add cssnano into postcss to shrink css output (#903)

- using [cssnano](https://github.com/cssnano/cssnano) can lower CSS build files, not by much but still is an improvement

packages/common/package.json

@@ -57,7 +57,7 @@
     "sass-build-task:scss-compile:salesforce-bare": "sass src/styles/slickgrid-theme-salesforce.bare.scss dist/styles/css/slickgrid-theme-salesforce.bare.css --style=compressed --quiet-deps --no-source-map",
     "sass-build-task:scss-compile:salesforce-lite": "sass src/styles/slickgrid-theme-salesforce.lite.scss dist/styles/css/slickgrid-theme-salesforce.lite.css --style=compressed --quiet-deps --no-source-map",
     "sass:build": "run-p sass-build-task:scss-compile:*",
-    "postsass:build": "postcss --no-map --use autoprefixer --dir dist/styles/css dist/styles/css --style=compressed --quiet-deps --no-source-map",
+    "postsass:build": "postcss --no-map --use cssnano --use autoprefixer --dir dist/styles/css dist/styles/css --style=compressed --quiet-deps --no-source-map",
     "presass:copy": "cross-env copyfiles -f src/styles/*.png dist/styles/css",
     "sass:copy": "cross-env copyfiles -f src/styles/*.scss dist/styles/sass",
     "postsass:copy": "cross-env copyfiles -f src/styles/*.png dist/styles/sass",
@@ -76,6 +76,7 @@
     "@slickgrid-universal/event-pub-sub": "workspace:~",
     "@slickgrid-universal/utils": "workspace:~",
     "autocompleter": "^8.0.2",
+    "cssnano": "^5.1.14",
     "dequal": "^2.0.3",
     "dompurify": "^2.4.3",
     "flatpickr": "^4.6.13",