lsp: Add a registry for client side code action commands

* preserve fields from LSP diagnostics via adding a user_data table to the diagnostic, which can hold arbitrary data in addition to the lsp diagnostic information.

This builds on https://github.com/neovim/neovim/pull/14112 and closes
https://github.com/neovim/neovim/issues/12326

This is mostly motivated by https://github.com/neovim/neovim/issues/12326

Client side commands might need to access the original request
parameters.

Currently this is already possible by using closures with
`vim.lsp.buf_request`, but the global handlers so far couldn't access
the request parameters.

- Fix the cursor position after applying TextEdits
- Support reversed range of TextEdit
- Invoke nvim_buf_set_text one by one

These links were actually defined backwards: the highlight groups
actually being used for display are the new "Diagnostic*" groups, so
linking the old "LspDiagnostics*" groups to these does absolutely
nothing, since there is nothing actually being highlighted with the
LspDiagnostics* groups.

These links were made in an attempt to preserve backward compatibility
with existing colorschemes. We could reverse the links to maintain this
preservation, but then that disallows us from actually defining default
values for the new highlight groups.

Instead, just remove the links and be done with the old LspDiagnostics*
highlight groups.

This is not technically a breaking change: the breaking change already
happened in #15585, but this PR just makes that explicit.

This generalizes diagnostic handling outside of just the scope of LSP.
LSP clients are now a specific case of a diagnostic producer, but the
diagnostic subsystem is decoupled from the LSP subsystem (or will be,
eventually).

More discussion at [1].

[1]: https://github.com/neovim/neovim/pull/15585

Fixes #15522