Fix prettier usage (#1172)

After some discussion in #1166, I realized that our prettier workflow is somewhat broken! This PR fixes it, by:

- separating Stylelint & Prettier (and removing `stylelint-prettier`) to clearly separate linting versus formatting 
    - this is also helpful for the upcoming upgrading of Stylelint
- having Prettier ignore `assets/js/zzzz-search-data.json`; on `main`, `npm run format` fails because of this file (Prettier doesn't know how to interpret the front matter)
- change `npm test` to run both Stylelint and Prettier on CI (uses `npm-run-all` utility
    - had to add one `// prettier-ignore` in `_includes/mermaid_config.js`, which I think is reasonably placed
- removing an unused `script` (that doesn't work!)

This is a no-op on the site itself, just for our dev environment/CI.

Ref: https://github.com/just-the-docs/just-the-docs/pull/1166#discussion_r1110397592.

---

Separately, upgrading to Stylelint 15 will come with another interaction - the [deprecation of stylistic rules](https://stylelint.io/migration-guide/to-15#deprecated-stylistic-rules) + shift to Prettier should be nice!

.prettierignore

@@ -1,4 +1,3 @@
-package.json
 package-lock.json
 _site
 assets/css/just-the-docs-default.scss
@@ -6,5 +5,6 @@ assets/css/just-the-docs-light.scss
 assets/css/just-the-docs-dark.scss
 assets/js/vendor/lunr.min.js
 assets/js/search-data.json
+assets/js/zzzz-search-data.json
 assets/js/just-the-docs.js
 *.md

.stylelintrc.json

@@ -9,9 +9,7 @@
     "stylelint-config-standard-scss",
     "stylelint-config-prettier-scss"
   ],
-  "plugins": ["stylelint-prettier"],
   "rules": {
-    "prettier/prettier": true,
     "alpha-value-notation": null,
     "color-function-notation": null,
     "no-descending-specificity": null,

_includes/mermaid_config.js

@@ -1 +1,2 @@
+// prettier-ignore
 {}

package.json

@@ -6,15 +6,17 @@
   "license": "MIT",
   "bugs": "https://github.com/just-the-docs/just-the-docs/issues",
   "devDependencies": {
+    "npm-run-all": "^4.1.5",
     "prettier": "^2.8.4",
     "stylelint": "^14.16.1",
     "stylelint-config-prettier-scss": "0.0.1",
-    "stylelint-config-standard-scss": "^6.1.0",
-    "stylelint-prettier": "^2.0.0"
+    "stylelint-config-standard-scss": "^6.1.0"
   },
   "scripts": {
-    "test": "stylelint '**/*.scss'",
+    "lint": "npm-run-all --parallel --continue-on-error lint:*",
+    "lint:css": "stylelint '**/*.scss'",
+    "lint:formatting": "prettier --check '**/*.{scss,js,json}'",
     "format": "prettier --write '**/*.{scss,js,json}'",
-    "stylelint-check": "stylelint-config-prettier-check"
+    "test": "npm run lint"
   }
 }