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/help/comment.md

@@ -47,6 +47,7 @@ but it is only available for certain filetypes:
 * java: `// %s`
 * javascript: `// %s`
 * jinja2: `{# %s #}`
+* json: `// %s`
 * julia: `# %s`
 * kotlin: `// %s`
 * lua: `-- %s`