The `split()` VimL function trims empty items from the returned list by
default, so that, e.g.

  split("\nhello\nworld\n\n", "\n")

returns

  ["hello", "world"]

The Lua implementation of vim.split does not do this. For example,

  vim.split("\nhello\nworld\n\n", "\n")

returns

  {'', 'hello', 'world', '', ''}

Add an optional parameter to the vim.split function that, when true,
trims these empty elements from the front and back of the returned
table. This is only possible for vim.split and not vim.gsplit; because
vim.gsplit is an iterator, there is no way for it to know if the current
item is the last non-empty item.

Note that in order to preserve backward compatibility, the parameter for
the Lua vim.split function is `trimempty`, while the VimL function uses
`keepempty` (i.e. they are opposites). This means there is a disconnect
between these two functions that may surprise users.

By remembering the keys already compared in repeating a comparison is
avoided. Thanks: https://stackoverflow.com/a/32660766

The official developer documentation in in :h dev-lua-doc specifies to
use "--@" for special/magic tokens. However, this format is not
consistent with EmmyLua notation (used by some Lua language servers) nor
with the C version of the magic docstring tokens which use three comment
characters.

Further, the code base is currently split between usage of "--@",
"---@", and "--- @". In an effort to remain consistent, change all Lua
magic tokens to use "---@" and update the developer documentation
accordingly.

An empty table was previously always treated as a list, which means that
while merging tables, whenever an empty table was encountered it would
  always truncate any table on the left.

`vim.tbl_deep_extend("force", { b = { a = 1 } }, { b = {} })`

Before: `{ b = {} }`
After: `{ b = { a = 1 } }`

* Implementation derived from and validated by vim-lsc authored by Nate
  Bosch

Breaking Changes:
- Deprecated all `vim.lsp.util.{*diagnostics*}()` functions.
    - Instead, all functions must be found in vim.lsp.diagnostic
    - For now, they issue a warning ONCE per neovim session. In a
      "little while" we will remove them completely.
- `vim.lsp.callbacks` has moved to `vim.lsp.handlers`.
    - For a "little while" we will just redirect `vim.lsp.callbacks` to
      `vim.lsp.handlers`. However, we will remove this at some point, so
      it is recommended that you change all of your references to
      `callbacks` into `handlers`.
    - This also means that for functions like |vim.lsp.start_client()|
      and similar, keyword style arguments have moved from "callbacks"
      to "handlers". Once again, these are currently being forward, but
      will cease to be forwarded in a "little while".
- Changed the highlight groups for LspDiagnostic highlight as they were
  inconsistently named.
    - For more information, see |lsp-highlight-diagnostics|
- Changed the sign group names as well, to be consistent with
  |lsp-highlight-diagnostics|

General Enhancements:
- Rewrote much of the getting started help document for lsp. It also
  provides a much nicer configuration strategy, so as to not recommend
  globally overwriting builtin neovim mappings.

LSP Enhancements:
- Introduced the concept of |lsp-handlers| which will allow much better
  customization for users without having to copy & paste entire files /
  functions / etc.

Diagnostic Enhancements:
- "goto next diagnostic" |vim.lsp.diagnostic.goto_next()|
- "goto prev diagnostic" |vim.lsp.diagnostic.goto_prev()|
    - For each of the gotos, auto open diagnostics is available as a
      configuration option
- Configurable diagnostic handling:
    - See |vim.lsp.diagnostic.on_publish_diagnostics()|
    - Delay display until after insert mode
    - Configure signs
    - Configure virtual text
    - Configure underline
- Set the location list with the buffers diagnostics.
    - See |vim.lsp.diagnostic.set_loclist()|
- Better performance for getting counts and line diagnostics
    - They are now cached on save, to enhance lookups.
    - Particularly useful for checking in statusline, etc.
- Actual testing :)
    - See ./test/functional/plugin/lsp/diagnostic_spec.lua
- Added `guisp` for underline highlighting

NOTE: "a little while" means enough time to feel like most plugins and
plugin authors have had a chance to refactor their code to use the
updated calls. Then we will remove them completely. There is no need to
keep them, because we don't have any released version of neovim that
exposes these APIs. I'm trying to be nice to people following HEAD :)

Co-authored: [Twitch Chat 2020](https://twitch.tv/teej_dv)

- The previous commit lost information in the tests. Instead, add some
  more "normalization" substitutions in pcall_err(), so that the general
  shape of the stacktrace is included in the asserted text.
- Eliminate contains(), it is redundant with matches()