Add example for Uppercase and Lowercase ASCII using builtins

See zulip discussion for context

We should add a new example which shows people how to make conversions to/from Upper and Lower case using only the builtins.

We should briefly explain the trade-offs, and how there will be packages available to provide additional functionality. Could even point people towards https://github.com/Hasnep/roc-ascii

Below is a quick and dirty implementation... we should modify this to handle the Result correctly, and polish it a little with some explanation and examples.

module [to_upper, to_lower]

to_upper : Str -> Str
to_upper = \str ->
    str
    |> Str.toUtf8
    |> List.map to_upper_help
    |> Str.fromUtf8
    |> unwrap

to_lower : Str -> Str
to_lower = \str ->
    str
    |> Str.toUtf8
    |> List.map to_lower_help
    |> Str.fromUtf8
    |> unwrap

expect to_upper "hello" == "HELLO"
expect to_upper "Hello" == "HELLO"

expect to_lower "HELLO" == "hello"
expect to_lower "Hello" == "hello"

to_upper_help = \c -> if c >= 'a' && c <= 'z' then c - delta else c
to_lower_help = \c -> if c >= 'A' && c <= 'Z' then c + delta else c
delta = 'a' - 'A'

unwrap : Result a _ -> a
unwrap = \result ->
    when result is
        Ok a -> a
        Err _ -> crash "quick and dirty error handling"

Start with the quick-and-dirty Roc implementation included in the issue and review the linked Zulip discussion for context. The example is done when it demonstrates uppercase and lowercase ASCII conversion using builtins, handles the Result correctly, and briefly explains trade-offs and available packages.