Add support for an `extensions` array in the `fmt` command

[kytrinyx]

We are adding an extensions array to the tracks config.json file. We will use this for a couple of different purposes:

• syntax highlighting - the Prism library can use file extensions to determine which syntax highlighting lexer to use
• inferring files to submit with CLI - the CLI can submit an exercise directory, but this often contains temp files, build files, artifacts, etc that we don't care to upload. The extensions will help us decide which files should be submitted

Let's add a field named Extensions (in JSON: extensions) to the Config type directly after the embedded PatternGroup. https://github.com/exercism/configlet/blob/d7e3715157cc66669afcd6322ad19b11fdf830c4/track/config.go#L41-L52

https://github.com/exercism/meta/issues/107

[nywilken]

@kytrinyx what else are we thinking here regarding support for Extensions. Is this a field we want to lint for as well. If we are going to go with the whitelisting functionality in the CLI I would think so just want to confirm.

[kytrinyx]

Yeah, I think we should lint for it.