emit table column alignments into Render JSON

[QuietMisdreavus]

Bug/issue #, if applicable: rdar://77749746

Summary

This PR takes the table alignment info from Swift-Markdown and emits it into Render JSON, so that Markdown tables' column alignment information can be reflected in the final rendering.

Dependencies

swiftlang/swift-docc-render#441

Testing

Use the following table as test information:

| Column One | Column Two | Column Three | Column Four |
| :--------- | ---------: | :----------: | ----------- |
| one        | two        | three        | four        |

Steps:

1. Add the above Markdown to Sources/SwiftDocC/SwiftDocC.docc/SwiftDocC.md.
2. Clone and build the Swift-DocC-Render PR.
3. DOCC_HTML_DIR=/path/to/swift-docc-render/dist bin/preview-docs
4. Ensure that the table's columns have the proper alignment when rendered.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• [ n/a ] Updated documentation if necessary

[QuietMisdreavus]

@swift-ci Please test

[franklinsch]

It seems odd to not assign to the property if they're all unset here. Is there a technical reason that's needed? As a user of the API, I'd expect my argument to get assigned to the property.

[QuietMisdreavus]

This is mainly for backwards compatibility with existing DocC-Render - the absence of the alignments list is the same as when they're all unset, so i decided to drop it to save on encoding space.

[franklinsch]

Not emitting it in the encoder seems to be handled in the encode function though, right? It doesn't seem like the extra check here is required.

[QuietMisdreavus]

Without this check, the assertRoundTripCoding would fail, because [.unset, .unset...] is not equal to nil. I could change the Equatable conformance to allow these things to compare equal, but i decided to have the check here as well to use the default conformance instead.

[franklinsch]

I see, then it seems like we should implement this filtering in the decoder, rather than the initializer, that way clients that use the Swift model directly aren't affected by this.

[QuietMisdreavus]

I added some filtering to the decoder and left the initializer alone, to allow all the means of creating the Table struct to get the same results. I've also added some comments describing the behavior of the initializer.

Didn't meant to hit the approval button

[QuietMisdreavus]

@swift-ci Please test

[QuietMisdreavus]

@swift-ci Please test

[franklinsch]

This could be inferred as this type's Equatable conformance implementing this behavior. Maybe we can mention that this is the behavior in practice or something like that.

[franklinsch]

Or we can actually override == and implement the behavior there.

[Kyle-Ye]

Should a Table (with 3 rows and alignments are [.unset, .unset, .unset]) equal to a Table (with the same 3 rows but alignments are nil)?

Their actual behavior is the same but the current default Equatable implementation is false.

[QuietMisdreavus]

I've pushed up a new commit that implements this behavior into the Equatable conformance, and adds some tests to ensure that the behavior holds.

[QuietMisdreavus]

@swift-ci Please test

[QuietMisdreavus]

@swift-ci Please test