Fintekkers-ledger-models

Adding support for displayValue from position.ts so that backends can summarize a model object for display purposes.

(Later we will support serialization/deserialization for svelte apps, which may require JSON serialization support)

There have been a couple cases where unknown enum values get into the database, presumably they're not being set. The validation logic on the backend needs improvement to capture this, so adding the ability for them to pass through to positions so that it doesn't break on deserialization and the data issue is visible to the user