feat(gatsby): support ?. and ?? in generated pages

[pvdz]

After this change you should be able to use the new ?. and ?? operators at runtime (so in the resulting site). They will be transformed by Babel.

Basically tested with an empty repo in dev mode, added console.log(SEO?.foo ?? 'bar') before and after this change. Before it would crash on the new syntax and afterwards it would work. Both develop and build.

[pieh]

Looking good. Updated readme to mention newly added plugins.

Not a blocker - do you think this warrants some e2e tests using new syntaxes to verify it works and catch potential regressions in future? This could be different PR.

I did manually verify that things work as expected, so this is good to go in as-is IMO

[pvdz]

do you think this warrants some e2e tests using new syntaxes

I considered it. I've been in this position before (with many many more babel plugins) and in that case I had a bunch of those tests. But the e2e were so expensive relative to the syntax never really changing that those tests were discarded after some time. So in this case I kind of decided that checking whether they worked would suffice.

At some point these syntax features will end up in e2e tests organically and they'll be picked up. (We can synthetically speed that process up as a side effect without creating explicit new tests for it, I guess ;) )

[pvdz]

Put differently, it would basically be testing whether Babel works.