Added Dart and Flutter snippets.

[Davoodeh]

I collected (and edited) these from Dart-Code (for VSC) and
YASnippet-snippets and wrote some of them myself.
This is the complete collection in here.

If there's gonna be a Dart-Mode in Doom. Also, this needs to be right
next to it.

I haven't contributed to Doom before this.
Originally I didn't intend to do such and wanted to make a package and add
it to the lang/dart/package.el in the Doom-Emacs. But since the pack wasn't
"innovative" I am not able to push it to MELPA, that's why the files are here.
So pardon me if I didn't suppose to open this.

[Davoodeh]

I configured lang/dart/package.el to install the snippets directly from my GH.
If there was necessary for them to be merged in Doom, I'll reopen this and remove my package from the dart/package.el

[hlissner]

Actually, I'd prefer that you PR these snippets here rather than add it to the module.

[Davoodeh]

Certainly :D
I'll remove them again and commit here again.
Later on after the merge I'll repropuse them.