`no-immediate-mutation`: option to allow big values

[fisker]

Description

Maybe sometimes an assignment should be preferred due to the code size.

For example

const object = {}
object.foo = function () {
  // the body only needs to indent one level
}

const object = {
  foo() {
    // now requires 2-level indentation
  }
}

A real world case prettier/prettier@9047a40

[fisker]

Since Object.assign() won't reduce indentation level, the option should not apply to it.

[sindresorhus]

What do you define big though?

[fisker]

Lines of code.