Support {gather} and {gather*} (#2183)

* Support `gather` and `gather*`

* Add bogus screenshots

* Add correct screenshots

* Fix typo

* Update screenshots

* Improve error handling

* Scope styles inside .katex{}

* Revert console.log to console.error

test/screenshotter/ss_data.yaml

@@ -141,6 +141,9 @@ FractionTest: |
          \xrightarrow[\dfrac g h]{\displaystyle\frac g h} \; \xrightarrow [2.\, \dfrac c d] {1.\, \displaystyle\frac c d}
     \end{array}
 Functions: \sin\cos\tan\ln\log
+Gather:
+    tex: \begin{gather} a=\frac 1 2 \\ e=b+c \end{gather}
+    display: 1
 Gathered: |
     \begin{gathered}
       x = \frac{1}{2} \\