* preserve fields from LSP diagnostics via adding a user_data table to the diagnostic, which can hold arbitrary data in addition to the lsp diagnostic information.

- main.c: remove os_setenv("MYVIMRC",…), it is already done by
  do_source().
  - This also sets $MYVIMRC to a full (absolute) path.
- code cleanup.

Some parts of LSP need to use cached diagnostics as sent from the LSP
server unmodified. Rather than fixing invalid line numbers when
diagnostics are first set, fix them when they are displayed to the user
(e.g. in show() or one of the get_next/get_prev family of functions).

Since the `State` is global, other scripts are unexpectedly affected during the
'inccommand' preview. This commit introduces a new flag for `do_cmdline`, in
order to ignore trailing '|'-separated commands only for the command invoking
the preview.

fix #8796, update #7494

* feat(diagnostic): add vim.diagnostic.match()
  Provide vim.diagnostic.match() to generate a diagnostic from a string and
  a Lua pattern.
* feat(diagnostic): add tolist() and fromlist()

Problem
- `redir_exec` is obsolete, but it keeps getting used in new tests
  because people copy existing tests.
- Disadvantages of `redir_exec`:
  - Captures extra junk before the actual error/message that we _want_ to test.
  - Does not fail on error, unlike e.g. `command()`.

Solution
- Use new functions like `nvim_exec` and `pcall_err`.

- Fix the cursor position after applying TextEdits
- Support reversed range of TextEdit
- Invoke nvim_buf_set_text one by one

fix(diagnostic): resolve nil bufnr in show_line_diagnostics

Problem:    Filler lines are wrong when changing text in diff mode.
Solution:   Don't change the filler lines on every change.  Check
            scrollbinding when updating the filler lines. (closes vim/vim#8809)
https://github.com/vim/vim/commit/04626c243c47af91c2580eaf23e12286180e0e81