repl: keep entries added while history file is loading

[mhayk]

Lines can be evaluated while the history file is still being read asynchronously by setupHistory(), e.g. when the input stream does not support pausing (programmatic REPLs driven by mock streams, sockets or pipes). The entries added to the in-memory history in the meantime were discarded once the file load completed, because the loaded entries overwrote the in-memory history, leaving the history file without those entries.

This change merges the persisted entries with the in-memory ones instead (newest first, capped at the configured history size).

Reproduction on main before this change:

import repl from 'node:repl';
import fs from 'node:fs';
import { Stream, Writable } from 'node:stream';

class In extends Stream { resume() {} pause() {} }
In.prototype.readable = true;

const file = '/tmp/history.txt';
fs.writeFileSync(file, 'persisted entry');

const r = repl.start({
  input: new In(),
  output: new Writable({ write(c, e, n) { n(); } }),
  terminal: true,
});
r.setupHistory(file, () => {
  console.log(r.history); // ['persisted entry'] — evaluated lines lost
});
r.input.emit('data', 'const a = 1\nconst b = 2\n');

After this change, r.history is ['const b = 2', 'const a = 1', 'persisted entry'] and the file is flushed accordingly.

Refs: #64508