patch 9.2.0471: vimvars di_key initialized at runtime

patch 9.2.0471: vimvars di_key initialized at runtime

Problem:  evalvars_init() copies each vimvar's name into di_key at
          startup and runtime-checks that the name fits in
          DICTITEM16_KEY_LEN, even though all names are known at
          compile time.
Solution: Embed the name in di_key via the VV_NAME macro so the
          initialization happens at compile time.  Drop the
          runtime length check and the STRCPY loop (John Marriott).

closes: #20185

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>