The current 'clamp_line_numbers' implementation modifies diagnostics in
place, which can have adverse downstream side effects. Before clamping
line numbers, make a copy of the diagnostic. This commit also merges the
'clamp_line_numbers' method into a new 'get_diagnostics' local function
which also implements the more general "get" method. The public
'vim.diagnostic.get()' API now just uses this function (without
clamping). This has the added benefit that other internal API functions
that need to use get() no longer have to go through vim.validate.

Finally, reorganize the source code a bit by grouping all of the data
structures together near the top of the file.

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

* improve handling of multi-byte deletions

the prior signature did not assume an active language client
this function can now be used directly by passing an offset encoding
defaults to utf-16 (standard for LSP)

If a LSP server sent a workspace edit containing a rename the buffers
file name changed without the server receiving a close notification for
the old buffer and without the client properly re-attaching on the new
file.

This affected `Move` code-actions in nvim-jdtls, but also
`vim.lsp.buf.rename` on a class level.

* handle when workspaceFolder is nil in buf.list_workspace_folders and buf.add_workspace_folder

* 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