Don't regenerate regexp string on every call

[wermipls]

As of current, regexp() in EmojiText creates a string from scratch on *every single call*. Since the method is invoked on every replace call, it results in a massive performance penalty when doing a lot of replace operations. This PR makes it so the string gets initialized once and reused on subsequent calls, resulting in as much as a 5-6x speedup on my machine.

Below is a simple test case. I chose to perform 500k replacements to exacerbate the difference, but even in a real application context the difference was still quite pronounced doing as little as 5k replacements on short to moderate length strings.

<?php

include __DIR__.'/vendor/autoload.php';

$et = new Astrotomic\Twemoji\EmojiText("Yo Angelo 🗿");

for ($i = 0; $i < 500000; $i++) {
    $et->toHtml();
}

Before the patch:

$ time php perf_test.php

real    0m9.726s
user    0m6.747s
sys     0m2.913s

After the patch:

$ time php perf_test.php

real    0m1.705s
user    0m1.695s
sys     0m0.010s