Configure offset encoding on apply_text_edits

[teddylear]

With the addition of adding the offset encoding to apply_text_edits, we set the default to utf-16 to make tests pass in the current state. However, we may want to set this via lsp as noted on this PR using vim.lsp.buf_get_clients(bufnr)[1].offset_encoding. Then we could default back to utf-16 if lsp not present (or set to some other default via configuration?). This would however mean that we would need an lsp in the CI, which would be interesting to add back in...