Add `string.strip_prefix` and `string.strip_suffix`

[BrewingWeasel]

These functions would return an Ok value with the prefix or suffix stripped, or an Error(Nil) value when the prefix or suffix does not exist in the string.

case string.strip_prefix("https://gleam.run", "https://") {
  Ok(site) -> "found site " <> site
  Error(Nil) -> "no site found" 
}
// -> "found site gleam.run"

(I know this can be done with adding strings in pattern matching, but this would be useful in pipes and there is no way to handle strip_suffix with that approach)

You can already achieve this through combining string.starts_with and string.drop_start, but that's rather clunky and this feels much more gleam-y to me.

I find myself using them a decent amount when I write rust, and I noticed they're even used several times in the gleam compiler!

[lpil]

Sounds good to me!

Do we want to consider any of the other designs for this? Elixir has replace functions instead I believe.

[silby]

Haskell text also has this API, it's good.