Parse ?// as ? and // outside destructuring patterns (#3593)

The lexer returned a single ALTERNATION token for `?//`, so any `?`
followed by `//` was consumed by it, and `.name?//"unknown"` failed
to parse even though they read as the optional operator followed by
the alternative operator.

Drop the lexer rule and spell the destructuring alternative operator
as `'?' "//"` instead. So `x ?// y` now parses as `x? // y`, while
`. as [$a] ?// [$b] | ...` keeps its meaning. Note that `?` and `//`
may now be separated by whitespaces inside a pattern.

tests/jq.test

@@ -1485,6 +1485,11 @@ try error(0) // 1
 null
 1
 
+# Lex token test #3589, like error(0)? // 1
+error(0) ?// 1
+null
+1
+
 1, try error(2), 3
 null
 1