Use SPDX identifier in license field of META6.json

[samcv]

Use the standardized identifier for the license field.
For more details see https://design.perl6.org/S22.html#license

[teodozjan]

I think it would be nice to add newline at EOF

[toolforger]

Another nitpick (or maybe not, don't know what the Perl6 take on these things is):
It would be nice to put the reformatting into one commit, the rearrangement into another, and the actual change into a third.
I was unable to see what was actually done, if I were to review this I'd be in trouble.

[samcv]

Ran my 2.0 version script on it. The 1.0 just read json and then wrote it back, the new one inserts in and spaces the license tag as well.