fix: print statistics placeholders fail in custom G-code (#16348)

The G-code editor lists the Print Statistics placeholders for every
custom G-code field, but total_weight, used_filament, total_cost,
print_time, normal_print_time, silent_print_time,
total_wipe_tower_cost and total_wipe_tower_filament were only defined
for filename_format. Using one in custom G-code, including the File
header G-code, stopped slicing with "Not a variable name".

They are known only once the whole G-code is generated, so they use the
reserved-tag mechanism of print_time_sec and used_filament_length: the
placeholder parser gets a tag such as @TOTAL_WEIGHT@ and
GCodeProcessor::run_post_process() replaces it with the same value
filename_format gets, with numbers written to two decimals. The
per-tag replacement loops are folded into one helper. The running
extruded_* totals are unchanged.

Fixes #15320
Fixes #7870

src/libslic3r/GCode/GCodeProcessor.hpp

@@ -601,6 +601,12 @@ inline constexpr float DEFAULT_FILAMENT_DENSITY = 1.245f;
             Print_Time_Minute_Placeholder,
             Print_Time_Sec_Placeholder,
             Used_Filament_Length_Placeholder,
+            Normal_Print_Time_Placeholder,
+            Silent_Print_Time_Placeholder,
+            Total_Weight_Placeholder,
+            Total_Cost_Placeholder,
+            Total_Wipe_Tower_Cost_Placeholder,
+            Total_Wipe_Tower_Filament_Placeholder,
         };
 
         static const std::string& reserved_tag(ETags tag) { return s_IsBBLPrinter ? Reserved_Tags[static_cast<unsigned char>(tag)] : Reserved_Tags_compatible[static_cast<unsigned char>(tag)]; }