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

tests/checks/cmdsub.fish

@@ -0,0 +1,44 @@
+#RUN: %fish %s
+
+# Command substitution inside double quotes strips trailing newline.
+echo "a$(echo b)c"
+# CHECK: abc
+
+# Nesting
+echo "$(echo "$(echo a)")"
+# CHECK: a
+echo "$(echo $(echo b))"
+# CHECK: b
+
+echo "$(echo multiple).$(echo command).$(echo substitutions)"
+# CHECK: multiple.command.substitutions
+
+test -n "$()" || echo "empty list is interpolated to empty string"
+# CHECK: empty list is interpolated to empty string
+
+# Variables in command substitution output are not expanded.
+echo "$(echo \~ \$HOME)"
+# CHECK: ~ $HOME
+
+echo "$(printf %s 'quoted command substitution multiline output
+line 2
+line 3
+')"
+# CHECK: quoted command substitution multiline output
+# CHECK: line 2
+# CHECK: line 3
+
+echo trim any newlines "$(echo \n\n\n)" after cmdsub
+#CHECK: trim any newlines  after cmdsub
+
+echo i{1, (echo 2), "$(echo 3)"}
+# CHECK: i1 i2 i3
+
+echo "$(echo index\nrange\nexpansion)[2]"
+#CHECK: range
+
+echo "$(echo '"')"
+#CHECK: "
+
+echo "$(echo $(echo 1) ())"
+#CHECK: 1