asciidoc: purify and add asciidocFull attribute

The current asciidoc expression is impure; it relies on several tools to
be found in PATH at runtime. This commit adds a enableStandardFeatures
parameter that, if true, pulls in all dependencies and patches asciidoc
to contain full paths to the tools.

I've set enableStandardFeatures = false for the existing asciidoc
attribute so that the closure size stays unchanged, at 255 MiB. The new
asciidocFull attribute (with enableStandardFeatures = true) has a
closure size of 1.5 GiB.