Add TypeScript support & make externals configurable

[georgyangelov]

This adds a configuration option for using TypeScript in projects.
The first use-case will be scratch-gui where we'd like to use TS for configuration interfaces that would be exposed to scratch-web.

Making a project use TS also involves including a tsconfig.json and the typescript dependency in the project itself, but the change here enables it to be processed by webpack. The ts-loader is the main thing here which transforms TS -> JS. Then the code is processed as before using Babel. Babel is usually not necessary given TS can process things like downleveling code and JSX conversion, but we have a lot of pure-js code to take care of.

I've configured the ts-loader to also does JSX conversion (requires the "jsx": "react" option in the tsconfig of each project) because otherwise they conflict on the React import (TS removes it as it thinks it's type-only, then Babel does not have it). It should be pretty safe to let TS do that for TS files.

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Additionally, I've added an addExternals function so that we can safely add new externals without overriding any default ones.

@cwillisf I was careful not to change behavior for projects with enableTS = false, but may have missed something. Which builds do you think make sense to test better?

[cwillisf]

The exclude clause sent me down quite a path! I guess we need a more careful not clause to ensure that Babel doesn't process things like node_modules/cat-blocks/msg/scratch_msgs.js, like you said, but does process things like node_modules/scratch-vm/src/virtual-machine.js. I haven't yet tried this in a monorepo context, so I'm not sure if the exclude needs further modification for that...

[georgyangelov]

Also, @cwillisf what should I do to handle the versioning? This should probably be a minor version?

[cwillisf]

Also, @cwillisf what should I do to handle the versioning? This should probably be a minor version?

semantic-release will calculate a new version number based on the "conventional commit" tags on your commits. Since you have feat: but no ! or BREAKING CHANGE, it'll make a minor version bump.