Bug: readonly string field not inferred as literal type

[marvinhagemeister]

Reported on discord: https://discord.com/channels/684898665143206084/1203185670508515399/1270065229497106565

I believe I found a bug with JSR's dts generation. namely, readonly string fields in classes don't get correctly inferred as literal types

// input
class DocNotFound extends Data.Error {
  readonly _tag = "DocNotFound"
}

// generated type:
class DocNotFound extends Data.Error {
  readonly _tag: string
}

// expected:
class DocNotFound extends Data.Error {
  readonly _tag: "DocNotFound"
}