Add custom literals example

[ageron]

This PR adds an example showing how to define custom literals using:

• from_numeral
• from_quote
• from_interpolation

The example shows the two syntaxes:

temp1 : Celsius
temp1 = 21

temp2 = 21.Celsius

It also shows:

• How these custom literals interact with (and differ from) functions like from_str.
• How to run any pure function at runtime using Ok(validated_value) = some_pure_function(...).

Fixes roc-lang/roc#10872