Fix autolink rendering in djot-reader.lua

[karlb]

The pandoc reader had no handlers for url and email AST nodes emitted by the parser for autolinks (<https://example.com> / <foo@example.com>), so any document containing one aborted with "attempt to call a nil value".

Both nodes carry .destination and string-content children — the exact shape Renderer:link already consumes — so aliasing is sufficient. Closes #21.

I didn't see a natural place to fit a test for this. The following would work, but feels a bit weird:

diff --git a/Makefile b/Makefile
index 727ee79..f0aef94 100644
--- a/Makefile
+++ b/Makefile
@@ -24,6 +24,11 @@ ci: testall install
        pandoc -t djot-writer.lua m.txt -o m.dj
        pandoc -f djot-reader.lua m.dj -o m.html
        rm m.dj m.html
+       printf 'See <https://example.com> and <foo@example.com>.\n' \
+         | pandoc -f djot-reader.lua -t html > autolinks.html
+       grep -q 'href="https://example.com"' autolinks.html
+       grep -q 'href="mailto:foo@example.com"' autolinks.html
+       rm autolinks.html
 .PHONY: ci
 
 fuzz:

[karlb]

The CI failures seem to be unrelated to this PR. I pushed #23 to fix the CI. That's LLM generated, but looks fine to me.