Changed vim.tbl_islist to vim.islist

[ColinKennedy]

Fixes this this warning in Neovim 0.10+

vim.tbl_islist is deprecated, use vim.islist instead. :help deprecated
Feature will be removed in Nvim 0.12
stack traceback:
        vim/shared.lua: in function 'tbl_islist'
        ...personal/.config/nvim/bundle/dial.nvim/lua/dial/util.lua:41: in function 'validate_list'
        ...onfig/nvim/bundle/dial.nvim/lua/dial/augend/constant.lua:39: in function 'new'
        ...onfig/nvim/bundle/dial.nvim/lua/dial/augend/constant.lua:133: in main chunk
        [C]: in function 'require'
        ...rsonal/.config/nvim/bundle/dial.nvim/lua/dial/augend.lua:2: in main chunk
        [C]: in function 'require'
        ...rsonal/.config/nvim/bundle/dial.nvim/lua/dial/config.lua:1: in main chunk
        [C]: in function 'require'
        ...sonal/.config/nvim/bundle/dial.nvim/lua/dial/command.lua:4: in main chunk
        ...
        vim/_editor.lua: in function 'cmd'
        ...local/share/nvim/lazy/lazy.nvim/lua/lazy/core/loader.lua:485: in function <...local/share/nvim/lazy/lazy.nvim/lua/lazy/core/loader.lua:484>
        [C]: in function 'xpcall'
        .../.local/share/nvim/lazy/lazy.nvim/lua/lazy/core/util.lua:113: in function 'try'
        ...local/share/nvim/lazy/lazy.nvim/lua/lazy/core/loader.lua:484: in function 'source'
        ...local/share/nvim/lazy/lazy.nvim/lua/lazy/core/loader.lua:443: in function 'source_runtime'
        ...local/share/nvim/lazy/lazy.nvim/lua/lazy/core/loader.lua:411: in function 'packadd'
        ...local/share/nvim/lazy/lazy.nvim/lua/lazy/core/loader.lua:346: in function '_load'
        ...local/share/nvim/lazy/lazy.nvim/lua/lazy/core/loader.lua:191: in function 'load'
        ...share/nvim/lazy/lazy.nvim/lua/lazy/core/handler/keys.lua:122: in function <...share/nvim/lazy/lazy.nvim/lua/lazy/core/handler/keys.lua:112>

This PR is compatible with Neovim version <0.9. If you don't need earlier versions using vim.islist should work as-is.

[monaqa]

Thank you!