Build TypeScript plugin with TypeScript 4

[benmccann]

Rollup Plugin Name: all

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

I don't expect this would cause folks issues in upgrading. The only code changes I had to make were slight and internal to the plugins, so I don't think the external types will change

Description

There are a number of open issues with the TypeScript plugin. I was poking around to see what would be required to fix some of them. Being on an older version of TypeScript makes things harder. E.g. it seems like there might be a bug in one of the TypeScript methods (#287 (comment)), but I can't report issues to the TypeScript team unless using the latest version of TypeScript. I also was trying to use an API that I saw in tsc and appeared to be exported, but VS Code is telling me it doesn't exist and I wonder if being on an older version of TypeScript might be why.

[shellscape]

Appreciate the effort here. For PRs that will affect more than one plugin, the changes should really be discussed ahead of time. Looks like I'm going to need to update the contributing document to cover this. PRs that affect multiple plugins should be rare and require a special commit history and a regular merge commit versus squash commit - this is to facilitate an independent plugin publishing process (versus babel where they publish the world for each new version).

We should hold off on this right now. If you'd like to update just the typescript plugin, roll the other plugins back, and repurpose this branch and PR for only that plugin, then I can get behind that. I leave that choice up to you.

[shellscape]

@NotWoods this was rebased to only affect the Typescript plugin, so it'll need your approval.