feat(lsp): support textDocument/onTypeFormatting (#34637)

Implements [on-type
formatting](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#textDocument_onTypeFormatting)
using a `vim.on_key()` approach to listen to typed keys. It will listen
to keys on the *left hand side* of mappings. The `on_key` callback is
cleared when detaching the last on-type formatting client. This feature
is disabled by default.

Co-authored-by: Maria José Solano <majosolano99@gmail.com>

runtime/lua/vim/lsp/client.lua

@@ -211,6 +211,9 @@ local all_clients = {}
 ---
 --- @field _enabled_capabilities table<vim.lsp.capability.Name, boolean?>
 ---
+--- Whether on-type formatting is enabled for this client.
+--- @field _otf_enabled boolean?
+---
 --- Track this so that we can escalate automatically if we've already tried a
 --- graceful shutdown
 --- @field private _graceful_shutdown_failed true?