Eclipse 4.13 support

[k-brooks]

Support for 4.13, please!
I'd be happy to contribute - I think I see what needs to be done based on (components of, not the default update) #423

[nedtwigg]

PRs are always welcome! Here's another example, a little simpler: https://github.com/diffplug/spotless/pull/384/files

[k-brooks]

As always - a speedy response @nedtwigg
Would you / diffplug prefer separate issues/PRs for the supporting _ext updates?

[k-brooks]

Actually, it looks like the version ranges in the _ext artifacts will permit what is necessary, disregard.

[k-brooks]

I take that back, the fat jar is bundling the pinned version.

[nedtwigg]

Bumping JDT shouldn't require any changes in _ext, #384 is a canonical example.

[k-brooks]

I was hoping to hit Groovy as well, we use it for Gradle and would like to have consistent eclipse versions, and figured I'd do the rest while I was there.
But can break that out if desired

[nedtwigg]

Ahh, roger. Combined or broken-out is fine, either way!