Add support for consumption by projects that use C++ modules

[netadr]

Adds a preprocessor define SAFETYHOOK_USE_CXXMODULES to public-facing headers that allows for the standard library to be included by-module instead of by-header to prevent issues in projects using C++ modules.

[cursey]

Can you add an option for this to cmake.toml. Something like the following:

[options]
# ...
SAFETYHOOK_USE_CXXMODULES = false

[conditions]
# ...
use-modules = "SAFETYHOOK_USE_CXXMODULES"

[target.safetyhook]
# ...
compile-definitions.use-modules = ["SAFETYHOOK_USE_CXXMODULES"]

This would let people opt into it via cmake options.

[netadr]

@cursey See if this is fine. I used inline CMake because cmkr doesn't have support for INTERFACE-visibility compile definitions, AFAIK.

[cursey]

LGTM