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.

Author: jcbrand

.prettierrc

@@ -0,0 +1,8 @@
+{
+  "arrowParens": "avoid",
+  "printWidth": 100,
+  "quoteProps": "preserve",
+  "singleQuote": true,
+  "spaceBeforeFunctionParen": true,
+  "tabWidth": 4
+}

package-lock.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",