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>

src/gen/gen_vimdoc.lua

@@ -286,6 +286,7 @@ local config = {
       'inline_completion.lua',
       'linked_editing_range.lua',
       'log.lua',
+      'on_type_formatting.lua',
       'rpc.lua',
       'semantic_tokens.lua',
       'tagfunc.lua',