fix: join newlines in label_description

[ofseed]

Some LSP servers return label_description containing newlines, which is not allowed since this part will be applied to

blink.cmp/lua/blink/cmp/windows/render/init.lua

Line 77 in 96191fd

vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, lines)

.

[Saghen]

Nice catch, thanks! I've switched this to use the newline character (↲) instead and backported that to label and label_detail as well.