Toml++ only ships toml.h (not toml.hpp) in versions older than 3.4

I'm using Debian 12 (Bookworm) with libtomlplusplus-dev (v3.3) and the build systems fails to include toml.hpp because the development library only ships toml.h and it is looking for toml.hpp

However, in Debian 13 (Trixe) and unstable, with version 3.4, they ship both header files.

It seems like it would be better to look for toml.h and possibly  have a compatibility symlink in the contrib directory that links to toml.hpp for consistency.

```
$ apt-cache policy libtomlplusplus-dev
libtomlplusplus-dev:
  Installed: 3.3.0+ds-1
  Candidate: 3.3.0+ds-1
  Version table:
 *** 3.3.0+ds-1 1090
       1090 https://deb.debian.org/debian bookworm/main amd64 Packages

$ ls /usr/include/toml++
.  ..  impl  toml.h

ERROR:
/build/zeal/src/libs/core/session.cpp:15:10: fatal error: toml++/toml.hpp: No such file or directory
   15 | #include <toml++/toml.hpp>
```

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