Add a Lua PDF generator

Publishing my old invoice layouter in a reusable scripting-based form,
rather than an annoyingly fixed binary.

Because Lua compiled for C++ might be hard to find, we provide a wrap.
Curiously, only GitHub releases seem to contain onelua.c,
which is a very handy file.

We could have also subprojected libqr, which is in the public domain,
however the other main dependencies are also LGPL like libqrencode is.
And it is likely to be installed.

The user manual also serves as a test.

README.adoc

@@ -33,6 +33,8 @@ Runtime dependencies: libcrypto (OpenSSL 1.1 API)
  $ cd builddir
  $ ninja
 
+Go
+~~
 In addition to the C++ version, also included is a native Go port,
 which has enhanced PDF 1.5 support:
 
@@ -56,6 +58,15 @@ Type=^PDF
 Open=%cd %p/extfs-pdf://
 ----
 
+Lua PDF generator
+~~~~~~~~~~~~~~~~~
+Build dependencies: Meson, a C++17 compiler, pkg-config +
+Runtime dependencies: C++ Lua >= 5.3 (custom Meson wrap fallback),
+ cairo >= 1.15.4, pangocairo, libqrencode
+
+This is a parasitic subproject located in the _lpg_ subdirectory.
+It will generate its own documentation.
+
 Contributing and Support
 ------------------------
 Use https://git.janouch.name/p/pdf-simple-sign to report bugs, request features,