Support `d2 validate file.d2 ...`

[acxz]

d2 validate (#2410) is a nice qol feature! @melsonic thanks for implementing this.

Considering that d2 fmt supports multiple files, it makes sense (and would support my usecase) that d2 validate also supports the same input syntax of multiple files.

Would be great if I could glob and just do d2 validate **/*.d2.

Due to the lack of this functionality, I'm currently using the following workaround:

for file in *.d2; do
  if [ -f "$file" ]; then
    d2 validate "$file"
  fi
done