LibCore+LibFileSystem: Move resolve_executable_from_environment to Core

Author: https://github.com/BenWiederhake
Commit: 26efdf9d4f
Pull-request: https://github.com/SerenityOS/serenity/pull/19135
Issue: https://github.com/SerenityOS/serenity/issues/18804
Reviewed-by: https://github.com/gmta ✅
Reviewed-by: https://github.com/timschumi

Userland/Libraries/LibCore/System.h

@@ -267,4 +267,6 @@ ErrorOr<AddressInfoVector> getaddrinfo(char const* nodename, char const* servnam
 ErrorOr<void> posix_fallocate(int fd, off_t offset, off_t length);
 #endif
 
+ErrorOr<String> resolve_executable_from_environment(StringView filename, int flags = 0);
+
 }