Ways to determine Elm version

[lassik]

Currently elm-format supports two mutually incompatible versions of Elm and needs to know which version code is written for.

elm-format can do this by:

1. Getting an explicit --elm-version command line option
2. Looking for elm.json or elm-package.json in the current directory (the policy of which directories to look in may change in the next version of elm-format, see auto-detect elm version per file, instead of based on cwd avh4/elm-format#561)

There is no syntax in the Elm language to declare the Elm version directly within a source file itself.

Notably, Unibeautify needs to support formatting unnamed blobs of code that are not saved into any file. For this, the only option would be --elm-version. We need to add an equivalent option to the Unibeautify configuration file. @Glavin001 @stevenzeck Opinions on where and how to do this?

(Ping @avh4 in case you are interested. Unibeautify may be the most complex/generic real world use case for elm-format since Unibeautify is very generic itself.)

[Glavin001]

Language version may become a first class feature for Unibeautify. Prettier's Trailing Commas option has an es5 option: https://prettier.io/docs/en/options.html#trailing-commas
Whereas Unibeautify's End With Comma option is boolean (only true or false): https://unibeautify.com/docs/option-end-with-comma.html?language=javascript

I would like to be able to specify the language version (like es5 versus es6 for JavaScript, Python 2 or 3, Elm version, etc) and without an options changing (such as end-with-comma) it will adapt appropriately.

cc @stevenzeck thoughts?

[lassik]

A generic language version option would be a great way to solve this Elm issue IMHO 😄