The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue #5074, "pages from other data sources".
But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.
Most notable changes:
* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
This improves the taxonomy template `.Title` situation and make common template constructs much simpler.
See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778
The type handling in these was improved in Hugo 0.49, but this also meant that it was no longer possible to start out with a string slice and later append `Page` etc. to it.
This commit makes sure that the old behaviour is now possible again by falling back to a `[]interface{}` as a last resort.
Fixes #5361
The fix introduced in Hugo `0.49.1` had an unintended side-effect in the `Append` func used in both `append` and `.Scratch.Add`.
This commit fixes that by loosen/fixing the type checking so concrete types can be appended to interface slices.
Fixes #5303
This commit consolidates the reflective collections handling in `.Scratch` vs the `tpl` package so they use the same code paths.
This commit also adds support for a corner case where a typed slice is appended to a nil or empty `[]interface{}`.
Fixes #5275
