Feature: add ability to pass in specific version of svelte

[saibotsivad]

This will resolve #123 by adding support for passing in a specific version of the Svelte compiler (more discussion on that issue).

My skills at TypeScript are sorely lacking, so I'm not confident that I added definitions correctly.

[antony]

Syntactical change

[lukeed]

Made the requested changes. Also made it so that the Svelte interface, which is now using version, is require-friendly. You can pass in individual keys, as was originally intended, or use require('svelte') directly. I updated the README to reflect this. @saibotsivad If you don't mind taking a peek at this again, would be great 🙇

[lukeed]

OK. We have decided that this will be backported as a 6.1.0 release... and then removed in the upcoming 7.0 release. The reasoning is that start with 7.0, we're dropping support for old Node/Rollup/Svelte versions. We have to do this for sanity, and semver allows us to do so (see #138)

Once I get the 6.x branch up to date, I will update this base branch & revert the rebase that I applied to this PR.