How to append to file before specific string in go?

I have a file that starts with this structure:

locals {
  MY_LIST = [
    "a",
    "b",
    "c",
    "d"
    //add more text before this
  ]
}

I want to add in the text "e" before the "//add more text before this" and "," after the "d" so it will be like this:

locals {
  MY_LIST = [
    "a",
    "b",
    "c",
    "d",
    "e"
    //add more text before this
  ]
}

How can I implement this dynamically so that I can add more strings to the file in the future?

Thanks