Add zstd fast levels and update 7z property sizes

- add the "fast compression levels" of zstd via "fast" option (fast=1..64)
- change the 7-Zip property sizes of LZ4, LZ5 and Zstandard to 3
- 3 and 5 byte header are valid now (default is 3)
- update the Methods-Extern.md file, to reflect the property changes

CPP/7zip/Common/MethodProps.cpp

@@ -281,6 +281,7 @@ static const CNameToPropID g_NameToPropID[] =
   { VT_BSTR, "filter" },
   { VT_UI8, "memuse" },
   { VT_UI4, "strat" },
+  { VT_UI4, "fast" },
   { VT_UI4, "long" },
   { VT_UI4, "wlog" },
   { VT_UI4, "hlog" },