Get rid of lowdown!

`cmark` is a more robust library for constructing Markdown than
`lowdown`. It in particular has the functionality we need to compute the
doc we need to compute. It is also more portable.

For terminal output, there is a new A.I. slop library has the same
license as the C code from lowdown upon which it is based. This keeps
our options open for upstreaming, if we want to do that.

Also, the `builtins.fetchTree` docs are now assembled using the cmark
AST rather than janky string indenting.

meson.build

@@ -13,6 +13,7 @@ project(
 )
 
 # Internal Libraries
+subproject('libcmarkcpp')
 subproject('libutil')
 subproject('libstore')
 subproject('libfetchers')