Problem:
`vim._with({ buf = 0, win = 0 }, f)` fails an assert in ctx_switch().
vim._with() allows `buf` with `win` when `win` shows `buf`, but
nlua_with() passes both, and ctx_switch() takes only one target.

Solution:
Ignore `buf` if `win` is given; the window implies its buffer.

Problem:
- buf-local CWD scope is lower priority than :lcd, which is weird.
  ```
  win > buf > tab > global
  ```
- No way to clear current CWD at a given scope.

Solution:
- Rerrange scope precedence to:
  ```
  buf > win > tab > global
  ```
- Introduce "bang" variants (`:bcd!`/`:lcd!`/`:tcd!`) which clears the
  local CWD for the given scope.

Problem:
- Explicit `:bcd` (etc.) persists from `nvim_buf_call()` but not from an
  autocmd handler targeting a hidden buf (`LspAttach`, `TermRequest`, …),
  which needs a `vim.schedule()` workaround.
- `vim._with()` is supposed to work as a "sandbox", discarding
  side-effects, but it leaks CWD changes: `:lcd` from a `win` context,
  any chdir from a visible-buffer context.

Solution:
- Explicit :cd/:tcd/:bcd during a temp context persists by default.
  - "Ambient" directory changes ('autochdir', existing win-local CWD,
    etc.) are still undone, as before.
- Add `kCtxKeepDirs`: snapshot/restore the target's full CWD state
  (w/b/tp-local, global, cwd). Used by `vim._with()` and `'inccommand'`,
  which must not leak state.

Problems:
1. Can't get individual parts of a key-chord separately: modifiers, key.
2. Can't separate a key-combo into individual key-chords.

Solution:
Enhance `vim.keycode()` to optionally return a structured parse result
as a list of key-chords:
- `key_raw` the key-chord (problem 2)
- `mod` the modifiers of `key_raw` (problem 1)
- `key_orig` the key part of the key-chord, only here if differing from `key`
  (this doesn't solve any of the above mentioned problems, but it may provide
  useful and it's (in terms of code) free)
- `key` a normalized version of `key_orig` (solving problem 1), example(the
  first is `key_orig` and second is `key`): `lt` and `<`, `Bar` and `|` (in
  `<C-Bar>`)

Problem:
`aucmd_restbuf` must be guarded in case `aucmd_prepbuf` wasn't called.

Solution:
Update `aucmd_restbuf` to be a no-op if `aucmd_prepbuf` wasn't called.
This requires `aco` to be zero-initialized.

Problem:
vim._with may silence all cmdline-errors if emsg_silent=true, silent=false.

Steps to reproduce:
`nvim --clean -u repro.lua`, then `:echoerr 123`, nothing is shown.

    local api, fn = vim.api, vim.fn
    -- api.nvim_create_autocmd("CursorMovedC", {
    api.nvim_create_autocmd("CmdlineChanged", {
      callback = function(args)
        if args.match ~= ":" then
          return
        end
        -- vim.cmd([[silent! ]])
        vim._with({ emsg_silent = true }, function()
          -- return api.nvim_parse_cmd(fn.getcmdline(), {})
        end)
      end,
    })

Solution:
Force CMOD_SILENT if CMOD_ERRSILENT.