feat(api): nvim_echo can emit Progress messages/events #34846

Problem:
Nvim does not have a core concept for indicating "progress" of
long-running tasks. The LspProgress event is specific to LSP.

Solution:
- `nvim_echo` can emit `kind="progress"` messages.
  - Emits a `Progress` event.
  - Includes new fields (id, status, percent) in the `msg_show` ui-event.
  - The UI is expected to overwrite any message having the same id.
- Messages have a globally unique ID.
  - `nvim_echo` returns the message ID.
- `nvim_echo(… {id=…})` updates existing messages.

Example:

    local grp = vim.api.nvim_create_augroup("Msg", {clear = true})
    vim.api.nvim_create_autocmd('Progress', {
      pattern={"term"},
      group = grp,
      callback = function(ev)
        print(string.format('event fired: %s', vim.inspect(ev))..'\n')
      end
    })

    -- require('vim._extui').enable({enable=true, msg={target='msg', timeout=1000}})
    vim.api.nvim_echo({{'searching'}}, true, {kind='progress',  percent=80, status='running', title="terminal(ripgrep)"})
    local id = vim.api.nvim_echo({{'searching'}}, true, {kind='progress', status='running', percent=10, title="terminal(ripgrep)"})
    vim.api.nvim_echo({}, true, {id = id, kind='progress', percent=20, status = 'running', title='find tests'})
    vim.api.nvim_echo({}, true, {id = id, kind='progress', status='running', percent=70})
    vim.api.nvim_echo({{'complete'}}, true, {id = id, kind='progress', status='success', percent=100, title="find tests"})

Followups:
- Integrate with 'statusline' by listening to the Progress autocmd event.
- Integrate progress ui-event with `vim._extui`.

runtime/lua/vim/pack.lua

@@ -395,7 +395,8 @@ local function new_progress_report(title)
     local progress = kind == 'end' and 'done' or ('%3d%%'):format(percent)
     local details = (' %s %s'):format(title, fmt:format(...))
     local chunks = { { 'vim.pack', 'ModeMsg' }, { ': ' }, { progress, 'WarningMsg' }, { details } }
-    vim.api.nvim_echo(chunks, true, { kind = 'progress' })
+    -- TODO: need to add support for progress-messages api
+    api.nvim_echo(chunks, true, {})
     -- Force redraw to show installation progress during startup
     vim.cmd.redraw({ bang = true })
   end)