spec(developer): LDML Keyboard Designer 🗼

## Introduction

This is a holding issue for the Keyman Developer LDML keyboard designer. We'll be filling this in as we complete planning and design.

**Objective:** build a tool that allows intuitive and visual editing of LDML keyboards for all platforms.

There are many parts to this, including visual editors, transform editors, testing interfaces. The intent is to build this in Typescript in such a way that it can be integrated into Keyman Developer but also in the future run as a standalone web app.

## Related

* [Draft spec](https://docs.google.com/document/d/1wVICtmyYcaX639KILkOO88B7nEovgphVE7MbVUtlPYI/edit)
* #5077: spec: LDML to KVKS tool
* #5078: spec: LDML to keyman-touch-layout tool

## Blocks the following Issues
* #10192
* #10549
* #10622
* #10815
* #10552
* #11083

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**Keyman Developer:**
- OS: Windows (Web)
- Keyman version: target 15.0