build: enable MSVC level 3 warnings (#21934)

MSVC has 4 different warning levels: 1 (severe), 2 (significant), 3
(production quality) and 4 (informational). Enabling level 3 warnings
mostly revealed conversion problems, similar to GCC/clang -Wconversion
flag.

src/nvim/api/options.c

@@ -181,7 +181,7 @@ void nvim_set_option_value(String name, Object value, Dict(option) *opts, Error
 
   switch (value.type) {
   case kObjectTypeInteger:
-    numval = value.data.integer;
+    numval = (long)value.data.integer;
     break;
   case kObjectTypeBoolean:
     numval = value.data.boolean ? 1 : 0;