:messages query API, kill "Press ENTER" with fire

[justinmk]

Problem

"Press ENTER" prompt is obnoxious. Most messages do not require explicit user confirmation. No, not even error messages.

The prevalence of "Press ENTER" is mostly an accident because of vim's reliance on stale tty state to show a message.

Related:

• Remove 'Press ENTER or type command to continue' #1029
• :messages filtering, avoid duplicates, "provider" hook #10879

Expected behavior

:messages should be querable via an API.
All messages emitted by Nvim should have metadata allowing Nvim (and plugins etc) to better decide:

1. which messages genuinely need active user confirmation
2. which messages can be shown passively (in the cmdline/message area)
3. which messages need a pager
4. which messages should be shown for a minimum of X seconds, fading away after some time

Message metadata

• result (needs pager -- or actual buffer/floating window)
• error (needs message, and be sticky; user cannot silence)
  • most errors don't need confirm. they can be "sticky" by showing them in the cmdline/message area (which may get overwritten by another message, then show +3 errors in 'showcmd')
• confirm (invokes "Press ENTER" or something like it)
• info (needs message, but user can decide how it happens)
• progress (be sticky and replaceable)

Eliminate "Press ENTER" (for 99% of cases)

With the above, Nvim can do smart things like , instead of showing "Press ENTER", show +3 messages in the 'showcmd' area (e.g.) and the user can check them if they want (via a mapping like g< or g>).

"Press ENTER" should be very rare except for actually dangerous situations.
"Editing a readonly file" is not a dangerous situation.