Generate enum cast helper for Postgres

[jlnstrk]

Closes #104. Related: eoin-obrien/prisma-extension-kysely#72, kysely-org/kysely#198.

Hey! This change adds to the generated code a helper function that can be used to type-safely cast enum literals in PostgreSQL queries.

To give an example, a query that before might have looked like this:

const unit: TemperatureUnit = "CELSIUS";
kysely.selectFrom("temperatures")
    .where("unit", "=", sql<TemperatureUnit>`${unit}::temperature_unit` /* zero type safety here! */)
    .selectAll()

Can be written as follows with this extension:

const unit: TemperatureUnit = "CELSIUS";
kysely.selectFrom("temperatures")
    .where("unit", "=", castEnumValue(unit, "temperature_unit" /* type-checked! */))
    .selectAll()

The change is compatible with the support for Prisma's multiSchema feature introduced in #56.

Curious to hear if this is something you'd consider merging.