Detect and Load child documents to include in validation

[zkamvar]

This PR will detect and load child files declared in R Markdown files. This is important because with overview lessons, and the HPC carpentry transition plan, and other community lessons, child files are heavily relied on for content, but they are not currently validated. This PR will fix #135

This is paired with carpentries/sandpaper#513

Testing

You can test this by checking out the branch and running the tests inside your local clone of {pegboard} using The usethis pull request helpers:

usethis::pr_fetch(137)
devtools::test()

Background

Child documents are defined in R Markdown documents like so:

This content is from _a child document_:

```{r child="files/the-child.md"}
```

where files/the-child.md would be relative to the parent markdown file. It is possible for a child document to have

We implemented detecting first-order children for build purposes in carpentries/sandpaper#499, but they are not loaded for validation, which leads to issues like carpentries/workbench#68.

Implementation

This implements the strategy to detect direct children by searching the code blocks for the @child attribute and extracting the file names from there when loading the Episodes.

Then, in the Lesson object, for all the episodes, each of the child files are passed through a recursive function that will load the file, gather the children (if any) and then load and append the children to the child list, recording the parents and the build parents (the source Episodes that will run the child file). With the build parents, we can query a child file and know exactly what source files it builds.

There is a method in the Lesson object that will get the entire child lineage by recursively searching the $children element until it is empty. The purpose of this is to gather the files necessary to generate the hash for the file.

[zkamvar]

I plan to release this on Wednesday, so I will merge now