Support "$(cmd)" command substitution without line splitting

This adds a hack to the parser. Given a command

	echo "x$()y z"

we virtually insert double quotes before and after the command
substitution, so the command internally looks like

	echo "x"$()"y z"

This hack allows to reuse the existing logic for handling (recursive)
command substitutions.

This makes the quoting syntax more complex; external highlighters
should consider adding this if possible.

The upside (more Bash compatibility) seems worth it.

Closes #159

src/highlight.cpp

@@ -494,6 +494,9 @@ static size_t color_variable(const wchar_t *in, size_t in_len,
         wchar_t next = in[idx + 1];
         if (next == L'$' || valid_var_name_char(next)) {
             colors[idx] = highlight_role_t::operat;
+        } else if (next == L'(') {
+            colors[idx] = highlight_role_t::operat;
+            return idx + 1;
         } else {
             colors[idx] = highlight_role_t::error;
         }