Provide a 'format' option for virtual text and floating window previews
that allows the displayed text of a diagnostic to be customized.

lsp: Add a registry for client side code action commands

This function isn't compatible with including diagnostic sources when
"source" is "if_many" since it only has access to diagnostics for a
single line. Rather than having an inconsistent or incomplete interface,
make this function private. It is still exported as part of the module
for backward compatibility with vim.lsp.diagnostics, but it can
eventually be made into a local function.

Add an option to virtual text display and floating window previews to
include diagnostic source in the diagnostic message.

* 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 fixes the handler signature and also prevents n+1 requests firing
if there are multiple clients.

(The first `prepareCallHierarchy` handler is called once per client,
each invocation used `buf_request` to make more requests using *all*
clients)

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.

Some parts of LSP need to use cached diagnostics as sent from the LSP
server unmodified. Rather than fixing invalid line numbers when
diagnostics are first set, fix them when they are displayed to the user
(e.g. in show() or one of the get_next/get_prev family of functions).

* feat(diagnostic): add vim.diagnostic.match()
  Provide vim.diagnostic.match() to generate a diagnostic from a string and
  a Lua pattern.
* feat(diagnostic): add tolist() and fromlist()