LaTeX reader: better support for \xspace.

Previously we only supported it in inline contexts; now
we support it in all contexts, including math.

Partially addresses #7299.

test/command/4442.md

@@ -5,5 +5,5 @@
 ^D
 \newcommand{\myFruit}{Mango\xspace}
 
-Mango\xspace is the king of fruits.
+Mango is the king of fruits.
 ```

test/command/7299.md

@@ -0,0 +1,23 @@
+```
+% pandoc -f latex -t plain
+$1-{\ensuremath{r}\xspace}$
+^D
+1 − r
+```
+
+```
+% pandoc -f latex -t plain
+\newcommand{\foo}{Foo\xspace}
+
+$\text{\foo bar}$
+^D
+Foo bar
+```
+
+```
+% pandoc -f latex -t plain
+a\xspace b
+^D
+a b
+```
+