Xonsh: Nested parentheses highlight incorrectly (innermost-opening and outermost-closing)

[OJFord]

The highlighting of parentheses is badly matched in a command of the form:

execx($(foobar))

in which the $( and final - not the penultimate - ) are highlighted green.

[ghost]

hey @OJFord thanks for reporting. This happens with any parens:

[OJFord]

No problem, so it does - I just happened to notice it with exec :)

Changed title to reflect.

[gforsyth]

This is fixed on master thanks to @blahgeek's great work in #1711