libcmd/markdown: handle allocation errors in lowdown_term_rndr

We upgrade to lowdown 0.8.0 [1] which contains a fix/improvement to a
behavior mentioned in this issue thread [2] where a big part of
lowdown's API would just call exit(1) on allocation errors since that
is a satisfying behavior for the lowdown binary.

Now lowdown_term_rndr returns 0 if an allocation error occurred which we
check for in libcmd/markdown.cc.

Also the extern "C" { } wrapper around lowdown.h has been removed as it
is not necessary.

[1]: 6ca7c855a0/versions.xml (L987-L1006)
[2]: https://github.com/kristapsdz/lowdown/issues/45#issuecomment-756681153

src/libcmd/markdown.cc

@@ -3,9 +3,7 @@
 #include "finally.hh"
 
 #include <sys/queue.h>
-extern "C" {
 #include <lowdown.h>
-}
 
 namespace nix {
 
@@ -42,7 +40,9 @@ std::string renderMarkdownToTerminal(std::string_view markdown)
         throw Error("cannot allocate Markdown output buffer");
     Finally freeBuffer([&]() { lowdown_buf_free(buf); });
 
-    lowdown_term_rndr(buf, nullptr, renderer, node);
+    int rndr_res = lowdown_term_rndr(buf, nullptr, renderer, node);
+    if (!rndr_res)
+        throw Error("allocation error while rendering Markdown");
 
     return std::string(buf->data, buf->size);
 }