Enable json5 and json-stringify parsers by default

These two parsers were accidentally omitted from the default list.  In
particular, this broke the choice of parser for package.json files.
So add them to the default list.

This solves the original motivating factor for #92, but not #92
itself which is a request for enhanced debugging capabilities.

Original commit by @aspiers, re-committed here in an attempt to fix a
mysterious CI failure.

prettier-tests.el

@@ -45,6 +45,7 @@
                 (insert-file-contents original-file)
                 (setq buffer-file-name original-file)
                 (rename-buffer original-file)
+                (prettier--eval-file-if-exists "prepare.elisp")
                 (set-auto-mode)
                 (prettier--eval-file-if-exists "setup.elisp")
                 (prettier-prettify)

prettier.el

@@ -153,6 +153,8 @@ Requires Prettier 1.9+."
                                       html
                                       java
                                       json
+                                      json5
+                                      json-stringify
                                       less
                                       lua
                                       markdown

test-cases/json-stringify/a.original.json

@@ -0,0 +1,3 @@
+{
+  "keywords": ["mtg"]
+}

test-cases/json-stringify/a.prettier.json

@@ -0,0 +1,5 @@
+{
+  "keywords": [
+    "mtg"
+  ]
+}

test-cases/json-stringify/package.json

@@ -0,0 +1,5 @@
+{
+  "devDependencies": {
+    "prettier": "^2.2.1"
+  }
+}

test-cases/json-stringify/prepare.elisp

@@ -0,0 +1,7 @@
+(progn
+  (setq buffer-file-name (concat (file-name-directory buffer-file-name) "package.json"))
+  (json-mode))
+
+;; Local Variables:
+;; mode: emacs-lisp
+;; End: