Added support for bold italic symbols (#1011)

* added support for \boldsymbol

* added \boldsymbol tests

* added \boldsymbol screenshots

* added \bm, snapshot, updated screenshots

* updated snapshot for \boldsymbol

* properly display \imath and 2 in boldsymbol

* implemented \boldsymbol for +

* fix typo, remove boldsymbolLetters hardcode

* make comment about using Main-Bold more general

* fixed trailing space CI error

test/screenshotter/ss_data.yaml

@@ -57,6 +57,7 @@ BinCancellation: |
     \end{array}
 BinomTest: \dbinom{a}{b}\tbinom{a}{b}^{\binom{a}{b}+17}
 BoldSpacing: \mathbf{A}^2+\mathbf{B}_3*\mathscr{C}'
+BoldSymbol: \sum_{\boldsymbol{\alpha}}^{\boldsymbol{\beta}} \boldsymbol{\omega}+ \int_{\boldsymbol{\alpha}}^{\boldsymbol{\beta}} \boldsymbol{\Omega}+\boldsymbol{Ax2k\omega\Omega\imath+}
 Boxed: \boxed{F=ma} \quad \boxed{ac}\color{magenta}{\boxed{F}}\boxed{F=mg}
 Cases: |
     f(a,b)=\begin{cases}