Typescript-ifying turf-great-circle

[thomas-hervey]

This change is intended to typescript-ify the turf-great-circles module following the typescript-ifying discussion #2538.

Summary of changes:

• Converted /index.js into /index.ts
• Added additional tests to /test.ts to cover different start and end input types
• Added /lib/arc.d.ts including class GreatCircle

Template requirements:

• Is this a bug fix, new functionality, or a breaking change?
  • This is an update of the turf-great-circles module to use typescript.
• Have read and followed the steps for preparing a pull request.

[thomas-hervey]

This is my first (draft) PR to TurfJS in preparation for other enhancement requests to turf-great-circle.
@smallsaucepan given that you've begun typescript-ifying other modules, this PR should help contribute to this discussion #2538.

[smallsaucepan]

Fantastic. Thanks @thomas-hervey. Will take a look at this shortly.

[smallsaucepan]

@thomas-hervey it would be great if we could take the opportunity to do away with the whole lib/arc.* section and import as a third party module. There is https://www.npmjs.com/package/arc (which it looks like we've copy pasted into Turf at some point) but that doesn't have types.

Are you possibly in a position to submit types to DefinitelyTyped for the arc JS lib, so we can import it nicely into Turf as part of your PR? If not - and don't feel obliged - I'll give it a whirl and let you know when it's ready to integrate.