Add a command line option to print the Pancake news file

[IlmariReissumies]

The flag is --pancake_news. Close #1469

[mktnk3]

I suppose this assumes that NEWS.md is already updated for the feature being added. I wonder if there is a way to ensure this is done. For example, requiring a non-empty (markdown?) string describing the changes that gets concatenated to the content of NEWS.md for both command line printing and updating the file... won't work probably :D Not sure how to go about it, but maybe it's worth stating that "NEWS.md should be updated" somewhere...

[IlmariReissumies]

Yes that's a good point, this works so long as we're disciplined about updating NEWS.md when merging Pancake PRs with user-facing changes. But it doesn't do anything to enforce this discipline.

I think that's difficult to enforce entirely automatically, because there's no good check for figuring out which changes are user-facing. But there are things that would probably help. Maybe a github hook that checks if PRs make any changes in the pancake/ directory and encourages NEWS.md updates or makes you tick a box that it's not needed?

Even so I guess this at least gives people a lower bound on the features they have :)

[KurtWu10]

Is there a simple way to make it more machine-readable?

The motivation for the issue is to improve the compatibility of Pancake programs, i.e. to enable programmers to write a single Pancake program that selects language features based on availability, similar to feature test macros in C. In the existing form, I guess I would write another wrapper to retrieve the latest date in the news, and then use the date to query a pre-defined list of dates to know the supported features.

[mktnk3]

Even so I guess this at least gives people a lower bound on the features they have :)

I can't quite decide whether that is a plus or a potential cause for confusion, but either way, that only won't be good enough to close the issue. I guess we can keep looking into more complete solutions.

@KurtWu10 would you like to add something on the motivation to the issue #1469 ?