fix: attempt to call field '_trim' (a nil value) error

[notomo]

This pr fixes the following error on nightly.

Error executing vim.schedule lua callback: ...pack/opt/lsp_signature.nvim/lua/lsp_signature/helper.lua:393: attempt to call field '_trim' (a nil value)
stack traceback:
    ...pack/opt/lsp_signature.nvim/lua/lsp_signature/helper.lua:393: in function 'cal_pos'
    ...ptpack/opt/lsp_signature.nvim/lua/lsp_signature/init.lua:501: in function 'handler'
    /home/notomo/.local/nvim/share/nvim/runtime/lua/vim/lsp.lua:1505: in function ''
    vim/_editor.lua: in function <vim/_editor.lua:0>

📝

• vim.lsp.util._trim is not public function. _trim is removed on nightly.
• vim.split trimempty option is implemented from v.0.6.0: https://github.com/neovim/neovim/releases/tag/v0.6.0