Add Project.add helper

[alexdavid]

No description provided.

[soenkehahn]

Trying this out in both #393 and #397 it seems that this solves our use-cases there. I think there's two open questions:

• Is .add the best name? Some alternatives are .with, .modify, .withSelf. Any other ideas? I kind of like both .add and .with. Although, technically, this could be used to not add something but somehow modify the project in another way, including removing things. So maybe .add is not the best name.
• Is there a good way to provide a type alias for these plugins? Something like type Plugin<T extends ProjectData, U extends ProjectData> = (project: T) => U. It'd be nice to be able to say e.g. vite.addBuilder has type Plugin<{ node_modules: Package }, { viteBundle: Package }>. (Modulo names. I don't like the name Plugin, any better ideas?) But toying around with that now I think that providing a type synonym that works well is not that easy. So maybe this PR should include a type synonym and have some type tests for that?

[jkarni]

Is .add the best name? Some alternatives are .with, .modify, .withSelf. Any other ideas? I kind of like both .add and .with. Although, technically, this could be used to not add something but somehow modify the project in another way, including removing things. So maybe .add is not the best name.

How about apply? It seems the most accurate.

[soenkehahn]

How about apply? It seems the most accurate.

We now changed the api of .add (and Plugin) so that it actually doesn't allow removing fields from a Project, see

garn/ts/project.ts

Lines 222 to 230 in a515a1e

	add<T extends ProjectData, Additions>(
	this: T,
	fn: Plugin<Additions, T>,
	): Omit<T, keyof Additions> & Additions {
	return {
	...this,
	...fn(this),
	};
	},

. (Maybe it's possible by doing something weird, like setting a field to undefined, but it's certainly not an obvious way to use the API.) So I think the name .add makes a lot of sense and is consistent with the other functions that start with add.

[alexdavid]

This looks good to me too. Going to merge