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`.

src/nvim/lua/executor.c

@@ -966,7 +966,7 @@ static void nlua_print_event(void **argv)
   HlMessage msg = KV_INITIAL_VALUE;
   HlMessageChunk chunk = { { .data = argv[0], .size = (size_t)(intptr_t)argv[1] - 1 }, 0 };
   kv_push(msg, chunk);
-  msg_multihl(msg, "lua_print", true, false);
+  msg_multihl(INTEGER_OBJ(0), msg, "lua_print", true, false, NULL);
 }
 
 /// Print as a Vim message