docs(eval): fix fn.strchar types #37737

Problem: The following strchar functions have incorrect types:

strcharlen() - Currently any. Always returns an integer, including on
error

strcharpart() - The skipcc annotation does not specify that 0 and 1 are
valid. These inputs are required for vimscript usage. The current
return type is any, even though the function returns an empty string
on error

strchars() - The skipcc annotation does not specify that 0 and 1 are
valid

Solution: Update the problem types.

runtime/lua/vim/_meta/vimfn.lua

@@ -9573,7 +9573,7 @@ function vim.fn.str2nr(string, base) end
 --- Also see |strlen()|, |strdisplaywidth()| and |strwidth()|.
 ---
 --- @param string string
---- @return any
+--- @return integer
 function vim.fn.strcharlen(string) end
 
 --- Like |strpart()| but using character index and length instead
@@ -9593,8 +9593,8 @@ function vim.fn.strcharlen(string) end
 --- @param src string
 --- @param start integer
 --- @param len? integer
---- @param skipcc? boolean
---- @return any
+--- @param skipcc? 0|1|boolean
+--- @return string
 function vim.fn.strcharpart(src, start, len, skipcc) end
 
 --- The result is a Number, which is the number of characters
@@ -9626,7 +9626,7 @@ function vim.fn.strcharpart(src, start, len, skipcc) end
 --- <
 ---
 --- @param string string
---- @param skipcc? boolean
+--- @param skipcc? 0|1|boolean
 --- @return integer
 function vim.fn.strchars(string, skipcc) end