closes https://github.com/neovim/neovim/issues/16352

* improve handling of multi-byte deletions

* use codeunits/points instead of byte ranges when applicable
* take into account different file formats when computing range and
  sending text (dos, unix, and mac supported)
* add tests of incremental sync

Rather than treating virtual_text, signs, and underline specially,
introduce the concept of generic "handlers", of which those three are
simply the defaults bundled with Nvim. Handlers are called in
`vim.diagnostic.show()` and `vim.diagnostic.hide()` and are used to
handle how diagnostics are displayed.

Also adds a check against the server capabilities to fix
https://github.com/neovim/neovim/issues/15183

* 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.

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