\kern fixes, \hskip support, \TeX, \LaTeX, \KaTeX (#974)

* Refactor \kern, proper \mkern support, add \hskip

* Move \kern, \mkern into functions directory
* Add \hskip, \mskip support (but without supporting plus/minus)
* Properly separate \kern, \hskip from \mkern, \mskip.
  (The former work in both modes, and don't support mu units.
  The latter work only in math mode and only support mu units.)

* Render \kern etc. using MathML <mspace>

* Implement \TeX macro

* Implement \LaTeX

* New KaTeX logo \katex

* Rename hskip.js -> kern.js

* Tweak katex \kern to 0.16em

* \katex kern -.17em

* Compute A raise height in \LaTeX and \katex

* Switch mu unit errors to warnings

* LaTeX screenshot test

* Replace \KaTeX with macro definition

* Update screenshots with \KaTeX in them

* Fix font selection for \*TeX macros

test/screenshotter/ss_data.yaml

@@ -119,12 +119,15 @@ GroupMacros:
         \endExp: \egroup
     tex: \startExp a+b\endExp
 HorizontalBraces: \overbrace{\displaystyle{\oint_S{\vec E\cdot\hat n\,\mathrm d a}}}^\text{emf} = \underbrace{\frac{q_{\text{enc}}}{\varepsilon_0}}_{\text{charge}}
-KaTeX: \KaTeX
+KaTeX:
+    tex: \KaTeX
+    nolatex: \KaTeX not supported by LaTeX
 Kern:
     tex: \frac{a\kern{1em}b}{c}a\kern{1em}b\kern{1ex}c\kern{-0.25em}d
     nolatex: LaTeX fails to typeset this, “Missing number, treated as zero.”
 Lap: ab\mathllap{f}cd\mathrlap{g}hij\mathclap{k}lm \; ab\llap{f}cd\rlap{g}hij\clap{k}lm
 LargeRuleNumerator: \frac{\textcolor{blue}{\rule{1em}{2em}}}{x}
+LaTeX: \text{\LaTeX}, \text{\TeX}
 LeftRight: \left( x^2 \right) \left\{ x^{x^{x^{x^x}}} \right.
 LeftRightListStyling: a+\left(x+y\right)-x
 LeftRightMiddle: \left( x^2 \middle/ \right) \left\{ x^{x^{x^{x^x}}} \middle/ y \right.\left(x\middle|y\,\middle|\,z\right)