Bug: removal of trailing whitespaces in multiline strings

Expert seems to remove trailing whitespaces even within a string block, i.e.
```elixir
input = ~S"""
123 328 51 64
45 64 387 23
6 98 215 314
* + * +
"""
```
becomes:
```elixir
input = ~S"""
123 328 51 64
45 64 387 23
6 98 215 314
* + * +
"""
```
A multi-line string should not be touched

Note: I run into this issue when trying to complete this years [advent of code](https://adventofcode.com/2025/day/6) puzzle (it's an issue in part2, but the block is visible already in part1)