Switch to Typescript

Given that this project interfaces with C code, it would benefit from use of Typescript because it would be easier to model the GDAL API interfaces that this is designed to connect to.

There are also some defined interfaces that Loam uses for internal communication that would benefit from being able to be formally defined somewhere.

Start by surveying the JavaScript sources and the project’s GDAL and internal communication interfaces; the issue does not name specific files or tests. Done would mean the project uses TypeScript and those interfaces are formally defined, but the migration scope and validation steps need to be decided first.

Written by the indexing model from the issue text.