Mutable "pipeline assignment" operator

Related problem

I'm not sure if I have the naming right, but when dealing with config (or other mutable variables), the convenience of the += and ++= operators only goes so far. When dealing with, for instance, a record that needs a merge, you still have to resort to $variable = $variable | merge {}.

We currently have:

╭───┬──────────┬────────────────╮
│ # │ operator │      name      │
├───┼──────────┼────────────────┤
│ 0 │ +=       │ PlusAssign     │
│ 1 │ ++=      │ ConcatAssign   │
│ 2 │ -=       │ MinusAssign    │
│ 3 │ *=       │ MultiplyAssign │
│ 4 │ /=       │ DivideAssign   │
╰───┴──────────┴────────────────╯

Describe the solution you'd like

A PipeAssign operator, probably |= which would allow a RHS expression that would simply use the value of the LHS as pipeline input. For example:

mut foo = "foo"
$foo |= str upcase
$foo
# => FOO

This comes in handy with longer $env.config entries.

# Without operator:
$env.config.hooks.env_change = (
    $env.config.hooks.env_change
    | merge deep --strategy=append {
          PWD: [{|before,after| null }]
      }
)

# With operator
$env.config.hooks.env_change |= merge deep --strategy=append {
    PWD: [{|before,after| null }]
}

Describe alternatives you've considered

No response

Additional context and details

No response

The issue names no files, tests, or entry points. Begin by tracing the existing += and ++= assignment operators in the Nushell source, then establish the scope and acceptance cases for |= from the examples, including mutable variables and nested $env.config paths.