Use bb() and ii() for bold and italics respectively in equations

[HassanAkbar]

As mentioned here https://github.com/metanorma/stepmod2mn/issues/93#issuecomment-1805476482

We need to use bb() and ii() for bold and italics respectively in equations e.g

<eqn>  <b> &#955;</b><i>(u)</i> = <b>C</b> +R(cos(<i>u</i>)<b>x</b> + sin(<i>u</i>)<b>y</b>)  </eqn>

Will be converted to

[stem]
++++
bb(λ)ii((u)) = bb(C) +R(cosii((u))bb(x) + sin(ii(u))b(y))
++++