change thickmathspace in \bmod to 5/18em

[hbghlyj]

$\bmod$ is currently converted to

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mo lspace="thickmathspace" rspace="thickmathspace">mod</mo>
</math>

It seems that thickmathspace was removed from MathML core. It would be good to change thickmathspace in \bmod to 0.2777em In Firefox 102, the console will show a warning:

In Firefox Nightly, the console will show an error:

[dpvc]

I've made a PR to address the problem in the next release.