QuasiQuotes for ICU

[samhh]

This could be something fun to explore: https://serokell.io/blog/parser-combinators-in-haskell#bonus%3A-quasi-quotations

[icu|Hello {name}|]

Might be helpful for testing stuff like flattening where ideally we could supply a known valid message without risk of it failing the test due to a bad parse (where this moves the error in that case up a layer into GHC).

[samhh]

Could also consider an eDSL via free monads? More ergonomic with do notation than patching strings together.