Add prettierx and tentatively use it for some formatting

I'm using prettierx, which is a fork of prettier so that we can have
spaces before the brackets in a function declaration.

I don't like how restrictive prettier is concerning its configuration
and formatting.

- it messes up some indentation in specs/
- it removes a second newline when I specifically want to have two
- it doesn't allow spaces before brackets in function declarations (hence prettierx)
- it adds spaces for function parameters with default values

None of this is configurable.

So auto-formatting is not turned on by default, but it might be a useful
tool in some cases.

Fixes an issue where indentation was off in converse-core.js, causing
vim-sleuth to wrongly indent by three spaces only.

.prettierrc

+{
+  "arrowParens": "avoid",
+  "printWidth": 100,
+  "quoteProps": "preserve",
+  "singleQuote": true,
+  "spaceBeforeFunctionParen": true,
+  "tabWidth": 4
+}

package-lock.json

@@ -12120,6 +12120,12 @@
       "integrity": "sha1-IZMqVJ9eUv/ZqCf1cOBL5iqX2lQ=",
       "dev": true
     },
+    "prettierx": {
+      "version": "0.6.0",
+      "resolved": "https://registry.npmjs.org/prettierx/-/prettierx-0.6.0.tgz",
+      "integrity": "sha512-eXc8J1spKCAlmdLjS/5mr79IgGfhG8M7/mK9Y4KqTA+IBbwqJpOcCB5i7yltsmY/AM3HT7GflBbJQjr63UKlZg==",
+      "dev": true
+    },
     "private": {
       "version": "0.1.8",
       "resolved": "https://registry.npmjs.org/private/-/private-0.1.8.tgz",

package.json

@@ -90,6 +90,7 @@
     "node-sass": "^4.11.0",
     "npm": "^6.4.1",
     "po2json": "^0.4.4",
+    "prettierx": "^0.6.0",
     "requirejs": "2.3.5",
     "run-headless-chromium": "^0.1.1",
     "sass-loader": "^7.1.0",