meson.build: adds summary() about Lua implementation used

This MR adds a summary() to be printed regarding whether built-in or system Lua will be used and which version.

Two things stand out:

1. the subproject's lua_dep.name() for some reason returns 'null' - possibly an upstream bug or an issue with the wrapper but for now it's "solved" by using a ternary operator to check for that and replace 'null' string it with less scary 'unknown' so as not to confuse users. I have also asked, for now, informally in the mesonbuild matrix room but haven't had an answer yet.
2. docs/rst/installing-wireplumber.rst contains outdated information implying that it's a feature set to auto (first try system Lua) instead of a boolean false (always use subproject Lua) and that Lua version 5.3 is the only one supported and built-in.

Neither is likely to be worth holding this MR up on, since better reporting on Lua is still a win.