Babel: [7.0] move legacy-decorators into the repo, replacing current decorators

[hzoo]

https://github.com/loganfsmyth/babel-plugin-transform-decorators-legacy

Basically since the current decorators transform throws an error, we should just move the implementation (even though it's the older stage-1 spec) into the babel repo. We'll do this in Babel 7 so it doesn't cause issues for anyone

We'll also want to change stage-1 to refer to the current transform again (reverting the previous pr) but we can do that.

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• [ ] Comment below you are going to do this (for others to know)
• [ ] Read CONTRIBUTING.md
• [ ] remember to run make build after a source code change or better just run make watch in another terminal window to auto run babel while you are developing
• [ ] Check you are on 7.0 branch with git checkout 7.0 and then make a branch off of that to PR
• [ ] Copy the contents of https://github.com/loganfsmyth/babel-plugin-transform-decorators-legacy into the transform-decorators folder under the packages/ directory. Try to keep the structure of the current repo but then replacing with src/tests/readme and merge the correct things where possible.
• [ ] might need to change the structure of the tests to match how babel does things via fixtures, etc but we can help with this

[oleksandr-kuzmenko]

@hzoo I'd like to work on this, if you don't mind

[hzoo]

Sounds good! I would recommend checking out our slack community if you need help (or comment here)

[hzoo]

Done in https://github.com/babel/babel/issues/5290!!