deps: update fmt to latest version (#3403)

This updates `fmt` to the latest version and moves to just being a copy
of their repo to make updating easier (no editing their cmake / figuring
out which files to minimally include).

The motivation for this is now that we switched to C++ 20, there were a
ton of deprecated function usages that is going away in future compiler
versions. This gets rid of all those warnings.

goalc/compiler/compilation/Debug.cpp

@@ -4,7 +4,7 @@
 #include "goalc/compiler/Compiler.h"
 #include "goalc/debugger/disassemble.h"
 
-#include "third-party/fmt/core.h"
+#include "fmt/core.h"
 
 u32 Compiler::parse_address_spec(const goos::Object& form) {
   if (form.is_int()) {