Communicate with language servers in terms of UTF-16 coordinates

This required indexing UTF-16 positions in `Rope`. We tried opting
into the UTF-8 experimental support but it didn't seem to work
correctly and the standard is UTF-16 anyway.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/lsp/src/lib.rs

@@ -226,7 +226,6 @@ impl LanguageServer {
                 experimental: Some(json!({
                     "serverStatusNotification": true,
                 })),
-                offset_encoding: Some(vec!["utf-8".to_string()]),
                 ..Default::default()
             },
             trace: Default::default(),