Enable EmmyLua's assign-type-mismatch diagnostic after correcting the
runtime annotations and assignments it flags.

Correct option, tuple, and return types, and use separate locals when a
value changes representation. Add guards and narrow casts where needed
so the runtime passes the stricter check. Regenerate the affected docs.

AI-assisted

Enable checks for unused code, deprecated calls, return types, and
annotations.

Fix the warnings and replace the deprecated highlight function. Keep
exceptions where old APIs are still needed or the checker gets it wrong.

AI-assisted

Problem: options related to ui2 (like `fillchars` with `msgsep`) do not
  take effect if set during startup after enabling ui2.

Solution: explicitly check just after startup if relevant options were
  changed during startup.

Problem:
The cmdline_hide callback causes an unnecessary cursor move to the cmdline,
before the normal redraw updates the cursor back to the current window. This
appears as "flicker" when using plugins such as matchit (legacy ":" mappings
instead of "<cmd>" mappings).

Solution:
Skip the immediate redraw for cmdline_hide events. The normal redraw
still updates the cursor after the cmdline window is hidden.

Problem:
In order for ui2 to graduate to the main "messages ui" its configuration
needs to graduate into actual options.

Solution:
Migrate some of its config to 'messagesopt':
- "maxheight" (note: currently this is an integer treated as
  a "percentage"; if we want to support a row count we could allow
  values with units, like `"42%"`)
- "pager"
- "timeout"

Also improves error messages:

    messagesopt=hit-enter,history:500,bogus       E474: Unknown item 'bogus'
    messagesopt=hit-enter,history:500,progress:x  E474: 'progress' must be one of: , c
    messagesopt=hit-enter,history:abc             E474: 'history' requires a number

Problem:
`pager_char` compares the `keytrans()` result exactly. E.g. `pager_char
= "<cr>"` does not work, it expects `<CR>` (uppercase).

Solution:
Normalize `pager_char` in enable().

Problem:
`vim.on_key()` callbacks are invoked for keys that `getchar()` consumes,
which never reach the main loop, and there is no way to tell such a key
apart.  With ui2 the `on_key` handler activated after interactive
cmdline, handles it as `pager_char` (thus enters the ui2 pager).

Solution:
Skip `vim.on_key()` callbacks during `getchar()`/`getcharstr()`.

Co-authored-by: darkdi <rantovov5@gmail.com>
Co-authored-by: Qiaoxi Guo <28090444+breadtitor@users.noreply.github.com>
Co-authored-by: zaveshaa <zaveshaa@gmail.com>