Strange `x = y // z` precedence

[A4-Tacks]

Describe the bug
The alternate operator has a higher precedence than the assignment operator, which is more practical, and vice versa is not very reasonable

To Reproduce

$ jq -n '.[0]=null//2'
[
  null
]
$ jq -n '.[0]=(null//2)'
[
  2
]

Expected behavior

[
  2
]
[
  2
]

Environment (please complete the following information):

• OS and Version: Arch Linux
• jq version 1.8.0