vim-patch:9.1.0547: No way to get the arity of a Vim function (#29638)

Problem:  No way to get the arity of a Vim function
          (Austin Ziegler)
Solution: Enhance get() Vim script function to return the function
          argument info using get(func, "arity") (LemonBoy)

fixes: vim/vim#15097
closes: vim/vim#15109

48b7d05a4f

Co-authored-by: LemonBoy <thatlemon@gmail.com>

runtime/lua/vim/_meta/vimfn.lua

@@ -2523,12 +2523,25 @@ function vim.fn.get(blob, idx, default) end
 --- @return any
 function vim.fn.get(dict, key, default) end
 
---- Get item {what} from Funcref {func}.  Possible values for
+--- Get item {what} from |Funcref| {func}.  Possible values for
 --- {what} are:
----   "name"  The function name
----   "func"  The function
----   "dict"  The dictionary
----   "args"  The list with arguments
+---   "name"    The function name
+---   "func"    The function
+---   "dict"    The dictionary
+---   "args"    The list with arguments
+---   "arity"   A dictionary with information about the number of
+---       arguments accepted by the function (minus the
+---       {arglist}) with the following fields:
+---     required    the number of positional arguments
+---     optional    the number of optional arguments,
+---           in addition to the required ones
+---     varargs     |TRUE| if the function accepts a
+---           variable number of arguments |...|
+---
+---     Note: There is no error, if the {arglist} of
+---     the Funcref contains more arguments than the
+---     Funcref expects, it's not validated.
+---
 --- Returns zero on error.
 ---
 --- @param func function