Support \pmb (#1418)

* Support \pmb

* Fix lint error

* Add phony screenshots

* Screenshots take two

* Screenshots take three

* Simplify copies

* Move determination of mclass from handler to htmlBuilder

* Change to a macro definition

* Update screenshots

* Pick up review comments

* Clean up nesting of braces

* Update screenshot

test/screenshotter/ss_data.yaml

@@ -274,6 +274,7 @@ Phantom: \begin{array}{l}
             \dfrac{1+\phantom{x^{\blue{2}}} = x}{1+x^{\blue{2}} = x} \left(\vphantom{\int_t} zzz \right) \left( X \hphantom{\frac{\frac X X}{X}} \right)\\
             \text{a \phantom{123} b \hphantom{45678} c \vphantom{101112} d}
          \end{array}
+Pmb: \mu\pmb{\mu}\pmb{=}\mu\pmb{+}\mu
 PrimeSpacing: f'+f_2'+f^{f'}
 PrimeSuper: x'^2+x'''^2+x'^2_3+x_3'^2
 Raisebox: \frac{a}{a\raisebox{0.5em}{b}} \cdot \frac{a\raisebox{-0.5em}{b}}{a} \cdot \sqrt{a\raisebox{0.5em}{b}} \cdot \sqrt{a\raisebox{-0.5em}{b}} \cdot \sqrt{a\raisebox{0.5em}{b}\raisebox{-0.5em}{b}}