Use // comments for .json (#3388)

Add comments to `.json` files using `//` instead of the default `#`.

Even though JSON does not support comments, JSON5 and JSONC exist, so `//` is a much more sane default. It also improves the experience of editing micro's own config files.

runtime/plugins/comment/comment.lua

@@ -28,6 +28,7 @@ ft["ini"] = "; %s"
 ft["java"] = "// %s"
 ft["javascript"] = "// %s"
 ft["jinja2"] = "{# %s #}"
+ft["json"] = "// %s"
 ft["julia"] = "# %s"
 ft["kotlin"] = "// %s"
 ft["lua"] = "-- %s"