Adds a parser and serializer for the EcoStd Module Metadata format
RFC: https://github.com/ecostd/rfcs/pull/3
This adapts the existing experimental support for import std; to use
the new parser. The CMAKE_CXX_STDLIB_MODULES_JSON is now the canonical
variable for controlling how CMake discovers the stdlib module metadata,
and is used directly by compiler detection.
Toolchains can still override the __CMAKE::CXX## targets if they wish,
either in conjunction with CMAKE_CXX_STDLIB_MODULE_JSON or not. It is
possible to disable automatic detection of
CMAKE_CXX_STDLIB_MODULE_JSON by setting it to the empty string.
When available, the CMAKE_CXX_STDLIB_MODULE_JSON will be used to create
all requested C++ stdlibs which do not already have targets.
