Support some Unicode Mathematical Alphanumeric Symbols (#1232)

* Support some  Unicode Mathematical Alphanumeric Symbols

This PR adds support for some of the characters in Unicode range U+1D400 to U+1D7FF, [Mathematical Alphanumeric Symbols](https://www.unicode.org/charts/PDF/U1D400.pdf).  Specifically, it adds support for:
* A-Z a-z bold
* A-Z a-z bold italic
* A-Z a-z Fractur
* A-Z a-z sans-serif
* A-Z double struck
•	A-Z script

Addresses issue #1215 and parts of #260.

* Fix lint error

* Fix MathML, and pick up review comments

* Fix lint error

* Add text mode. Remove sans-serif.

* Fix lint error

* Fixed \mathrm, added screenshotter test

* Change screenshotter test to an array

* Add screenshots

* Picked up review comments. Add characters

test/screenshotter/ss_data.yaml

@@ -292,6 +292,12 @@ SupSubHorizSpacing: |
 SupSubLeftAlignReset: |
     \omega^8_{888} \quad \frac{1}{\hat{\omega}^{8}_{888}} \quad \displaystyle\sum_{\omega^{8}_{888}}
 SupSubOffsets: \displaystyle \int_{2+3}x f^{2+3}+3\lim_{2+3+4+5}f
+SurrogatePairs: |
+    \begin{array}{l}
+        𝐀𝐚𝑨𝒂𝔅𝔞𝔸𝒜 \\
+        \text{𝐀𝐚𝑨𝒂𝔅𝔞𝔸𝒜} \\
+        \mathrm{𝐀𝐚𝑨𝒂𝔅𝔞𝔸𝒜}
+    \end{array}
 SvgReset:
     # This math uses a lot of SVG glyphs
     tex: '\underbrace{\sqrt{\vec{x}} + \sqrt{\vec{\color{#f00}y}}}'