vim-patch:8.2.{2518,2520,3572,3588}: 'listchars' (and 'fillchars'?) fixes

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

* 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.

Problem:    "verbose pwd" shows confusing info when :lcd does not change
            directory.
Solution:   Clear last_chdir_reason also when the directory does not change.
            (closes vim/vim#9160)
https://github.com/vim/vim/commit/64be6aa3a54ecfe355d4a03e1200650c301e7f29

This only ports the tests, as this is already Nvim's behavior.

Problem:    getcwd() is unclear about how 'autochdir' is used.
Solution:   Update the help for getcwd().  Without any arguments always return
            the actual current directory. (closes vim/vim#9142)
https://github.com/vim/vim/commit/851c7a699ae00bdc14a4db874cf722b7b7393b53

Problem:    ":verbose pwd" does not mention 'autochdir' was applied.
Solution:   Remember the last chdir was done by 'autochdir'.  (issue vim/vim#9142)
https://github.com/vim/vim/commit/0526815c15170a5926e1008600ec29d42d8b64c2