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/__snapshots__/mathml-spec.js.snap

@@ -77,6 +77,46 @@ exports[`A MathML builder should make prime operators into <mo> nodes 1`] = `
 
 `;
 
+exports[`A MathML builder should render boldsymbol with the correct mathvariants 1`] = `
+
+<math>
+  <semantics>
+    <mrow>
+      <mrow>
+        <mi mathvariant="bold-italic">
+          A
+        </mi>
+        <mi mathvariant="bold-italic">
+          x
+        </mi>
+        <mn mathvariant="bold-italic">
+          2
+        </mn>
+        <mi mathvariant="bold-italic">
+          k
+        </mi>
+        <mi mathvariant="bold-italic">
+          ω
+        </mi>
+        <mi mathvariant="bold-italic">
+          Ω
+        </mi>
+        <mi mathvariant="bold-italic">
+          ı
+        </mi>
+        <mo mathvariant="bold-italic">
+          +
+        </mo>
+      </mrow>
+    </mrow>
+    <annotation encoding="application/x-tex">
+      \\boldsymbol{Ax2k\\omega\\Omega\\imath+}
+    </annotation>
+  </semantics>
+</math>
+
+`;
+
 exports[`A MathML builder should render mathchoice as if there was nothing 1`] = `
 
 <math>

test/mathml-spec.js

@@ -75,4 +75,9 @@ describe("A MathML builder", function() {
         expect(getMathML(`x_{y_{\\mathchoice{D}{T}{S}{${cmd}}}}`))
             .toMatchSnapshot();
     });
+
+    it("should render boldsymbol with the correct mathvariants", () => {
+        expect(getMathML(`\\boldsymbol{Ax2k\\omega\\Omega\\imath+}`))
+            .toMatchSnapshot();
+    });
 });

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}