Improve error message of vec_recycle_common()

[mgirlich]

When using the .size argument there is no information for the user where the size comes from, which can make it a bit confusing (e.g. in tidyverse/dplyr#6887)

vctrs::vec_recycle_common(x = 1:3, y = 2:3, .size = 2)
#> Error:
#> ! Can't recycle `x` (size 3) to size 2.
#> Backtrace:
#>     ▆
#>  1. ├─vctrs::vec_recycle_common(x = 1:3, y = 2:3, .size = 2)
#>  2. └─vctrs:::stop_recycle_incompatible_size(...)
#>  3.   └─vctrs:::stop_vctrs(...)
#>  4.     └─rlang::abort(message, class = c(class, "vctrs_error"), ..., call = call)
vctrs::vec_recycle_common(x = 1:3, y = 2:3, .size = 2, .arg = "arg")

I saw that there is the .arg argument but the effect was quite surprising to me:

#> Error:
#> ! Can't recycle `arg$x` (size 3) to size 2.
#> Backtrace:
#>     ▆
#>  1. ├─vctrs::vec_recycle_common(x = 1:3, y = 2:3, .size = 2, .arg = "arg")
#>  2. └─vctrs:::stop_recycle_incompatible_size(...)
#>  3.   └─vctrs:::stop_vctrs(...)
#>  4.     └─rlang::abort(message, class = c(class, "vctrs_error"), ..., call = call)

^{Created on 2023-07-21 with reprex v2.0.2}

1. Should .arg really be used the way it is? To me this feels quite surprising to suddenly get <.arg>$<dot name> though I definitely see the use cases.
2. Maybe add something like .size_arg that is mentioned in the error message.