Add babylon as default Prettier parser

[chaucerbao]

I'm not sure why the default let l:parser = 'babylon' was removed. babylon is the default parser for prettier.

But since it was removed, this fixer doesn't work for regular Javascript files. This PR will connect javascript file-types to the babylon parser, while keeping the current let l:parser = ''.

[chaucerbao]

I see an explanation in this comment, but I believe it's a wrong assumption. If there is no file extension, we need to explicitly set a parser, because prettier has no idea what type of file is being passed in.

So the two solutions are:

1. Set a default l:parser = 'babylon' to mimic prettier (solution before Javascript prettier filetype detect fix #1769).
2. Add 'javascript': 'babylon' to the map (this current PR Add babylon as default Prettier parser #2220).

[chaucerbao]

Actually, the first is a better solution. There's never a case, when there is no file extension, where you'd want prettier to auto-detect the file type, because it basically cannot. That's why there's a default parser.

I'll change this PR to reflect that.

[w0rp]

https://prettier.io/docs/en/options.html#parser It does seem like babylon has been renamed in newer versions.

[w0rp]

I wonder if we might have to use the newer name for the parser for newer prettier versions.

[chaucerbao]

@w0rp I added a version check to use babel for Prettier v1.16.0+, and added the test.

[w0rp]

Cheers! 🍻