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.

common/formatter/formatter_tree.cpp

@@ -2,7 +2,7 @@
 
 #include "common/util/string_util.h"
 
-#include "third-party/fmt/core.h"
+#include "fmt/core.h"
 
 std::string get_source_code(const std::string& source, const TSNode& node) {
   uint32_t start = ts_node_start_byte(node);