Support [if] shortcode fields recursively (magic-tags)

[JoryHogeveen]

Fixes: #5470

Description

Add support for recursive if shortcodes for relationship fields. Example:

• [if relationship.relationship_field] content [/if]
• [if relationship.sub_relationship.sub_relationship_field] content [/if]

TODO

• [if] (confirmed on Slack by Adam Spelbring): 2021-04-06)
• [each]

ChangeLog

• Enhancement: Support [if] and [each] shortcode tags recursively (magic-tags)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows has proper inline documentation.
• My code includes automated tests for PHP and/or JS (if applicable).

[JoryHogeveen]

@jimtrue @sc0ttkclark
Also see issue #5470

This PR will need a thorough testing round with all kings of shortcode and template usages (widgets??)
I'm not sure about the current extend of possibilities with this area of Pods so I can imagine I'm overlooking some features that need testing as well.

@sc0ttkclark especially this part: https://github.com/pods-framework/pods/pull/5471/files#diff-ab22a13affdb28c998776e73a1808abfR579-R585

[tr1b0t]

🚫 8 error(s)
⚠️ 2 warning(s)

[tr1b0t]

🚫 4 error(s)

[sc0ttkclark]

Were the changes at https://github.com/pods-framework/pods/pull/5471/files#diff-3af4ca77fc3cd86765f464bb6fa7000725503257e80ccc885464b3ad43ab36c4L536-L540 not enough to do what you wanted here, is this part of the code still necessary? It seems that the previous field() alone would do what this does

[JoryHogeveen]

No, currently only using the field method isn't enough. The field type and Pod object needs to be updated as well so traversals will work properly.
However, I do agree that this might not be the best way to do this.

[sc0ttkclark]

Needs more discussion on the field() workaround

[sc0ttkclark]

I recommend punting this and adding fields() support for traversal of relationship fields so that logic can be simplified

[sc0ttkclark]

Traversal is in Whatsit::get_field() right now, what else do we need to do here to leverage that?

[JoryHogeveen]

@sc0ttkclark Not sure, if you say traversal is already there then maybe this PR isn't needed anymore, or can at least get a big refactor.
It's been a long time since I worked on this, I'll need to revisit what I've done.