Errors were being caused by invalid buffers being kept around in
diagnostic_cache, so add a metatable to diagnostic_cache which attaches
to new buffers in the cache, removing them after they are invalidated.

Closes #16391.

Co-authored-by: Gregory Anders <8965202+gpanders@users.noreply.github.com>

* send vim.NIL instead of not sending workspaceFolders
* read fallback rootPath and rootUri from workspaceFolders
* update documentation

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

See:

- https://emmylua.github.io/annotations/param.html
- https://emmylua.github.io/annotations/return.html

If the quickfixlist item doesn't contain a column it is reported as 0.
Rather than using a nil value in such a case (which breaks diagnostics
elsewhere), just keep the 0 value.