bug: error on opening Namu with `vim.o.winborder = "bold"`

[ptdewey]

When opening Namu (with any sub-command other than help or colorscheme) with vim.o.winborder = "bold" set, the following error occurs:

vim.schedule callback: ...vim/site/pack/core/opt/namu.nvim/lua/namu/selecta/ui.lua:226: border chars must be one cell
stack traceback:
        [C]: in function 'nvim_open_win'
        ...vim/site/pack/core/opt/namu.nvim/lua/namu/selecta/ui.lua:226: in function 'create_prompt_window'
        ...vim/site/pack/core/opt/namu.nvim/lua/namu/selecta/ui.lua:1039: in function 'create_windows'
        ...ite/pack/core/opt/namu.nvim/lua/namu/selecta/selecta.lua:566: in function 'pick'
        ...pack/core/opt/namu.nvim/lua/namu/namu_workspace/impl.lua:365: in function 'callback'
        ...te/pack/core/opt/namu.nvim/lua/namu/namu_symbols/lsp.lua:185: in function 'handler'
        ...k.dewey/neovim/share/nvim/runtime/lua/vim/lsp/client.lua:735: in function ''
        vim/_core/editor.lua: in function <vim/_core/editor.lua:0>

Looking at the code, it seems like bold (and possibly other built-in winborder styles) would need to be added to get_prompt_border and get_border_with_footer in ui.lua.

nvim version: v0.12.0-dev-2205+g13a9cdc6b4

I'd be happy to look into this later and make a PR if it would be helpful.