Add option to prevent removing empty lines in getMultilineInput

Describe the enhancement

The getMultilineInput function currently uses a filter to remove empty lines from the input.

It would be convenient to disable this behavior since the empty lines could be from a pre-formatted string. (For example, my use case is creating a comment with output from Terraform. Since empty lines are removed, the output is much less readable).

There is a workaround, which is to use the getInput function, but it would be convenient to have this behavior built in to the getMultilineInput function itself.

Code Snippet

For example, if there was an option added called filterEmptyLines of type boolean, it could be used like

import * as core from "@actions/core"

const strWithEmptyLines = core.getMultilineInput("my_string", { filterEmptyLines: false })

Doing it this way will make the behavior opt-in and backwards compatible.

Happy to do a PR for this :)

Start at packages/core/src/core.ts around the getMultilineInput implementation linked in the issue. Review how its current filtering handles empty lines and the surrounding public API before checking the existing core package tests. Done means an option can preserve empty lines while the default behavior remains backward compatible.