Make Page an interface

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  gohugoio#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.

And it's faster.

See gohugoio#5074
Fixes gohugoio#5763
Fixes gohugoio#5758
Fixes gohugoio#5090
Fixes gohugoio#5204
Fixes gohugoio#4695
Fixes gohugoio#5607
Fixes gohugoio#5707
Fixes gohugoio#5719
Fixes gohugoio#3113
Fixes gohugoio#5706
Fixes gohugoio#5767
Fixes gohugoio#5723
Fixes gohugoio#5769
Fixes gohugoio#5770
Fixes gohugoio#5771
Fixes gohugoio#5759
Fixes gohugoio#5776
Fixes gohugoio#5777

benchbep.sh

@@ -0,0 +1,2 @@
+gobench -package=./hugolib -bench="BenchmarkSiteBuilding/TOML,num_langs=3,num_pages=5000,tags_per_page=5,shortcodes,render" -count=3 > 1.bench
+benchcmp -best 0.bench 1.bench