Add trim_prefix and trim_suffix in string.gleam

[gungun974]

See #743

This PR is still a draft and for now I only implement trim_prefix and trim_suffix that just trim the string without returning any error. (if there is no pattern, it's the same string as input)

(That's the same behavior and API from the golang strings package, https://pkg.go.dev/strings#TrimSuffix)

[gungun974]

I added strip_prefix and strip_suffix as there were the original goal of #743 which I misunderstood.

They do the similar thing as trim_prefix and strip_suffix however if the string don't have the prefix or suffix, a Error(Nil) is return which make possible for doing stuff with a case (for example adding a new prefix if the stripping was sucessfull otherwise return the original string)