Migrate the code generator to use work items GraphQL API

Description

A spike was conducted for this in the Haskell version at #36 (closed) and glql-haskell!54. Going forward, I believe we'll deprecate the issues API in favour of work items API, so it makes sense for GLQL to start working with the work items API instead.

Pre-requisites

Currently, the work items API lacks the feature to query by dates (created, updated, closed, due), health status, progress (for OKRs) and weight. To maintain parity with existing GLQL features, we should add that feature to the GraphQL API.

A separate issue should be created to track this.

Requirements

For this issue:

1. a new code generator should be created that targets the WorkItems GraphQL API.
2. a new data transformer (in javascript) should be created for work items widgets, that transforms each widget into a field that can be rendered in a table.

Note: we are also in process of migrating the fields parser and data transformers (currently in javascript) to Rust. This is being tracked in #27 (closed).