Error accepting suggestions with multibyte (Cyrillic) characters (`utf-16` -> `utf-8` fixes the issue)

[Galicarnax]

When I type with Cyrillic alphabet, and getting suggestions, pressing the key binding to accept gives this error:

Error  02:24:02 PM msg_show.lua_error Error executing vim.schedule lua callback: /usr/share/nvim/runtime/lua/vim/lsp/util.lua:485: index out of range
stack traceback:
	[C]: in function 'get_line_byte_from_position'
	/usr/share/nvim/runtime/lua/vim/lsp/util.lua:485: in function 'apply_text_edits'
	...l/share/nvim/lazy/copilot.lua/lua/copilot/suggestion.lua:489: in function ''
	vim/_editor.lua: in function <vim/_editor.lua:0>

Looking into lua/copilot/suggestion.lua, I see this:

    vim.lsp.util.apply_text_edits({ { range = range, newText = newText } }, vim.api.nvim_get_current_buf(), "utf-16")

If I change utf-16 to utf-8, everything seems to work fine. Any reason for utf-16 here?

[traysh]

I'm having the same problem when I have any accents in the line I'm accepting the suggestion, such as é or ö. I'm on Linux, the default encoding is utf-8.

I confirm setting that line to utf-8 fixes it for me, but it would probably break for files with other encodings, and some platforms use other encodings by default. Maybe it would be better to check the buffer encoding?

[traysh]

This PR should fix it.