LaTeX writer: separate successive quote chars with thin space

Successive quote characters are separated with a thin space to improve
readability and to prevent unwanted ligatures. Detection of these quotes
sometimes had failed if the second quote was nested in a span element.

Closes: #6958

test/command/6958.md

@@ -0,0 +1,10 @@
+Add thin space between single and double quotes.
+```
+% pandoc -t latex+smart
+---
+lang: en-GB
+---
+'["On the Outside"]{}: Constructing Cycling Citizenship.'
+^D
+`\,{``On the Outside''}: Constructing Cycling Citizenship.'
+```