backport: fix(vim.opt): vimL map string values not trimmed (#14982)

Options formatted as a list of comma-separated key-value pairs may have
values that contain leading and trailing whitespace characters. For
example, the `listchars` option has a default value of
`"tab:> ,trail:-,nbsp:+"`. When converting this value to a lua table,
leading and trailing whitespace should not be trimmed.

Co-authored-by: Robert Hrusecky <robert.hrusecky@utexas.edu>

test/functional/lua/vim_spec.lua

@@ -1332,12 +1332,12 @@ describe('lua stdlib', function()
 
       it('should work for key-value pair options', function()
         local listchars = exec_lua [[
-          vim.opt.listchars = "tab:>~,space:_"
+          vim.opt.listchars = "tab:> ,space:_"
           return vim.opt.listchars:get()
         ]]
 
         eq({
-          tab = ">~",
+          tab = "> ",
           space = "_",
         }, listchars)
       end)