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/_meta/api_keysets.lua

@@ -165,6 +165,7 @@ error('Cannot require a meta file')
 --- |'OptionSet'
 --- |'PackChanged'
 --- |'PackChangedPre'
+--- |'Progress'
 --- |'QuickFixCmdPost'
 --- |'QuickFixCmdPre'
 --- |'QuitPre'
@@ -233,6 +234,11 @@ error('Cannot require a meta file')
 --- @field err? boolean
 --- @field verbose? boolean
 --- @field kind? string
+--- @field id? integer|string
+--- @field title? string
+--- @field status? string
+--- @field percent? integer
+--- @field data? table<string,any>
 
 --- @class vim.api.keyset.empty