CI leg to build with langverion=preview

[vzarytovskii]

I expect it to fail at first, since we have some badly formatted tests in the Salsa suite and preview has the parser recovery changes, which promote it to error by default (with a way to opt-out).

Update: Tests are properly formatted in the last commit, so the CI won't fail on them with latest lang version (please note, that the failure is intentional, based on @auduchinok's parser recovery changes).
Update 2: FSharpLangVersion is used instead of LangVersion because it was failing for our only vbproj

[vzarytovskii]

@auduchinok just fyi - notice last commit, this was ignoring the warning about offsets, and failing with preview version of the language.

[auduchinok]

@auduchinok just fyi - notice last commit, this was ignoring the warning about offsets, and failing with preview version of the language.

Thanks! It seems it was relatively easy to make an error in the tree examples.

[auduchinok]

I'm a bit late to the party, but maybe we could have a separate CI leg with --strict-indentation+ from #15491 in addition to this one? There're many other features in the preview, and probably it would be great to see this change enabled separately from them too.

There's a failure in the preview leg, and the tree produced in TypedTreeOps looks correct to me:

Having a separate leg would make it much easier to spot the parsing regressions.