Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libcmd/markdown.hh

@@ -0,0 +1,17 @@
+#pragma once
+///@file
+
+#include <string_view>
+
+namespace nix {
+
+/**
+ * Render the given Markdown text to the terminal.
+ *
+ * If Nix is compiled without Markdown support, this function will return the input text as-is.
+ *
+ * The renderer takes into account the terminal width, and wraps text accordingly.
+ */
+std::string renderMarkdownToTerminal(std::string_view markdown);
+
+}