build(vendor): vendor libraries now install in a dedicated directory

We now install vendor libraries into includedir/fourdst/vendor to avoid conflicts with user installed libraries. Note that currently this only applies to headers and not compiled shared libraries

BREAKING CHANGE:

build-config/tomlpp/meson.build

@@ -2,6 +2,6 @@ tomlpp_dep = dependency('tomlplusplus', required: true, static: true).as_system(
 
 install_subdir(
     meson.project_source_root() / 'subprojects' / 'tomlplusplus-3.4.0' / 'include',
-    install_dir: get_option('includedir'),
+    install_dir: get_option('includedir') / 'fourdst' / 'vendor',
     strip_directory: true
 )