Enables the "Format" row in "playlistinfo" responses.

https://github.com/MusicPlayerDaemon/MPD/issues/1094

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1048

Will be used for Song objects representing tracks inside a CUE file.

This attribute is not a URI; it is just the filename without its
parent directory path.  To avoid confusion, let's rename it to
"filename", leaving the struct without a "uri" attribute.

No longer allocate it as a "VarSize".  This used to be a clever trick
to save memory 10 years ago, but these days, keeping the code
maintainable seems more important than saving a few kilobytes of
memory.

Allow forward-declaring it.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

This enforces the "not nullptr" rule.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Reduce overhead.  LightSong can always point to the "real" allocated
string.