feat(gatsby-plugin-typescript): add support for ECMAScript Private Fields

[Colt45s]

Description

Similar to #19302, TypeScript 3.8 beta has supported ECMAScript Private Fields but this is only beta. (not stable)

So I create PR for Draft.

[pvdz]

Just to be clear; this is not the same as ?. and ?? because those features are now actually part of the spec. They are essentially, now, "proper JS". As such we've made sure people can use them.

Private fields do not have this status yet. They are stage 3 (see https://tc39.es/proposal-class-fields/#prod-FieldDefinition). I don't know how this resonates to/in the TS world.

Beyond that, can you please tell us what you'd like to do with this draft PR? I'm not sure whether we'll merge this (leaving that up to @blainekasten and @ascorbic to decide) as long as its stage 3.

[ascorbic]

Thanks for this. I'd usually be wary of supporting anything at stage 3, but if this is in 3.8 without a flag then I think we should probably merge this when it's released, subjects to the caveats below. There's already native support in Chrome, and I don't think there's any danger of the syntax changing now.

Things we'd need to do beforehand though:

• Decide if we should use the loose option
• Check for eslint support

I don't think we should start using it in our own TS code until it's stage 4 though.

[LekoArts]

Feel free to reopen when this is actually stable, in the meantime I'll close this.