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>

vim-patch:8.2.{3468,3617,3618,3622}: some other CWD related patches

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.

Problem:    Problem with :cd when editing file in non-existent directory. (Yee
            Cheng Chin)
Solution:   Prepend the current directory to get the full path. (closes vim/vim#8903)
https://github.com/vim/vim/commit/c6376c798433bcb9ee38a8664299d11454546950

Make the bufnr argument have similar semantics across API functions;
namely, a nil value means "all buffers" while 0 means "current buffer".
This increases the flexibility of the API by allowing functions such as
enable() and disable() to apply globally or per-namespace, rather than
only on a specific buffer.

Rename the `show_header` option in `open_float` to simply `header` and
allow users to specify both the header string as well as the highlight
group.

Extend the 'prefix' option for `open_float` to also provide an optional
highlight group for the prefix string.

The 'prefix' option accepts a function or a string that is used to add a
prefix string to each diagnostic displayed in the floating window.