`vec_cast_common()` should gain `.to_arg`

[DavisVaughan]

I have a case where I know the type I'm casting .to, and that .to also has a known name, so I'd like to also supply .to_arg, like you already can with vec_cast() itself.

I am fairly certain that to make this work we will have to give vec_ptype_common() a .ptype_arg argument as well, because we end up supplying .to as .ptype and we will need to tweak the name used there.

library(vctrs)

x <- 1
# I'd like to be able to use this!
x_arg <- "this"

elements <- list(
  1L,
  2,
  "x"
)

vec_cast_common(
  !!!elements,
  .to = x,
  .arg = "elements"
)
#> Error:
#> ! Can't convert `elements[[3]]` <character> to <double>.

# Like this
vec_cast(
  elements[[3]],
  to = x,
  to_arg = x_arg
)
#> Error:
#> ! Can't convert `elements[[3]]` <character> to match type of `this` <double>.

^{Created on 2025-07-15 with reprex v2.1.1}