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/main.c

@@ -182,6 +182,9 @@ void early_init(mparm_T *paramp)
 #ifdef MSWIN
   OSVERSIONINFO ovi;
   ovi.dwOSVersionInfoSize = sizeof(ovi);
+  // Disable warning about GetVersionExA being deprecated. There doesn't seem to be a conventient
+  // replacement that doesn't add a ton of extra code as of writing this.
+# pragma warning(suppress : 4996)
   GetVersionEx(&ovi);
   snprintf(windowsVersion, sizeof(windowsVersion), "%d.%d",
            (int)ovi.dwMajorVersion, (int)ovi.dwMinorVersion);