Provide a standalone build of GUI and configure TS support

[georgyangelov]

• Added TS support so that and configuration interfaces we write can have exported types.
• Removed react, react-dom, redux and react-redux from the regular bundle (scratch-gui.js) as they weren't used by scratch-www anyway
• Added a separate output bundle called scratch-gui-standalone which includes all of the dependencies above and exposes a function to instantiate the editor with its own React instance. This allows scratch-web and scratch-gui to not conflict on React versions - one can use 16, the other can use 18 with no issues.

I haven't tested the build with scratch-www @MiroslavDionisiev what's the simplest way to do that? Want to make sure the externals were not actually used from scratch-www.

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I will make some changes to abstract away project loading soon, but that would probably be a separate PR

[georgyangelov]

The build fails as this PR depends on changes from scratchfoundation/scratch-webpack-configuration#54 which are not in NPM yet

[georgyangelov]

I realized I hadn't configured eslint to check TS files and corrected that.

@cwillisf does everything in scratch-webpack-configuration and here looks ok to you? Do we push the buttons? 😄

[cwillisf]

Looks good! Merge (both) when ready :)