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 #5074Fixes#5763Fixes#5758Fixes#5090Fixes#5204Fixes#4695Fixes#5607Fixes#5707Fixes#5719Fixes#3113Fixes#5706Fixes#5767Fixes#5723Fixes#5769Fixes#5770Fixes#5771Fixes#5759Fixes#5776Fixes#5777Fixes#5778
It is slightly slower, but correctnes is, of course, more important:
```bash
benchmark old ns/op new ns/op delta
BenchmarkSortByWeightAndReverse-4 367 645 +75.75%
benchmark old allocs new allocs delta
BenchmarkSortByWeightAndReverse-4 2 2 +0.00%
benchmark old bytes new bytes delta
BenchmarkSortByWeightAndReverse-4 64 64 +0.00%
```
Running the same benchmark without any cache (i.e. resorting the slice on every iteration) and then compare it to the current version shows that it still is plenty worth it:
```bash
▶ benchcmp 2.bench 1.bench
benchmark old ns/op new ns/op delta
BenchmarkSortByWeightAndReverse-4 1358757 645 -99.95%
benchmark old allocs new allocs delta
BenchmarkSortByWeightAndReverse-4 17159 2 -99.99%
benchmark old bytes new bytes delta
BenchmarkSortByWeightAndReverse-4 274573 64 -99.98%
```
Closes#5239
Currently it makes no practical difference, but this is more a protection if we in the future creates index from the content related fields. That will not work from a shortcode.
See #5071
This closes#98, even if this commit does not do full content text search.
We may revisit that problem in the future, but that deserves its own issue.
Fixes#98
