(MAINT) Initial publishing of FrontMatter configs (#44)

This change extends the functionality of memo for documenting a hugo
theme to enable a theme author to publish [FrontMatter][01] config
files, which users can then reference in their own projects instead of
having to find and copy all the values.

This change is in support of estruyf/vscode-front-matter#407, which adds
the `frontMatter.extends` key.

In this basic implementation:

- An author can define `memo.front_matter.configs` in their site params
or on a particular page to publish one or more configurations.

The implementation requires that the configurations be defined as
assets. The implementation allows you to specify the config path as a
remote asset, a site asset, or a page asset and resolves the config path
to the appropriate asset.

Authors can define either the `definition` or `merge` key in an item but
not both. Items with the `definition` key publish a single config and
have the option to specify the site-relative publish path with the
`publish` key. Items with the `merge` key merge the values from multiple
configs to publish as a merged blob to the site-relative path defined by
the mandatory `publish` key.
- Memo has been updated to take advantage of the merge-from-data feature
in Platen to allow setting the site parameters in a data file.
- The new configuration options are validated before use, warning and
discarding invalid values.

.frontmatter/config/content/pageFolders/posts.json

@@ -0,0 +1,7 @@
+{
+  "name": "posts",
+  "title": "Platen blog posts",
+  "$schema": "https://beta.frontmatter.codes/config/content.pagefolders.schema.json",
+  "path": "[[workspace]]/content/posts",
+  "contentTypes": ["posts"]
+}