Add 'configurable' property to class fields

[reznord]

Q	A
Fixed Issues	Fixes #6108
Patch: Bug Fix?	No
Major: Breaking Change?	No
Minor: New Feature?	No
Tests Added/Pass?	Yes
Spec Compliancy?	No
License	MIT
Doc PR
Any Dependency Changes?	No

Adds configurable: true key here: https://github.com/babel/babel/blob/7.0/packages/babel-plugin-transform-class-properties/src/index.js#L27

    Object.defineProperty(REF, KEY, {
      configurable: true,
      enumerable: true,
      writable: true,
      value: VALUE
    });

The below are the checklist mentioned in the issue:

• Comment below you are going to do this (for others to know)
• Read CONTRIBUTING.md
• Check you are on 7.0 branch with git checkout 7.0 and then make a branch off of that to PR
• Join our slack if you haven't already if you need more immediate help or just comment in this issue/PR (this might be down, we can add you manually)
• check the class properties package/folder
• follow the steps above in changing the behavior
• fix tests
• update the readme if necessary

[mention-bot]

@reznord, thanks for your PR! By analyzing the history of the files in this pull request, we identified @loganfsmyth, @existentialism and @hzoo to be potential reviewers.

[xtuc]

Awesome, nice first contribution 👍

[reznord]

There are a few tests that are left behind (7 tests) when I ran make test.

Any idea why that is happening? And the code-coverage also shows 86%. Is it normal?

Thanks @xtuc 😄