Fix parser to allow binary operators for binding syntax (#3326)

Previously, the precedence of `as` binding was higher than the binary
operators. This results in unexpected behaviors, `1 + 2 as $x | -$x`
was `-1` instead of `-3`. The commit 92daac9 (PR #3053) changed
the behavior to emit syntax error on this case. Now, this commit allows
binary operators with `as` binding. This fixes #2446, and fixes #3284.

Author: itchyny