feat(nx-plugin): add migration schematic

[devinshoemaker]

This change adds a new schematic that generates a migration for an Nx plugin. The user can specify the version for the migration, and optionally add a custom name, description, and whether or not to include package.json updates.

Examples:

nx g @nrwl/nx-plugin:migration --project=ionic-react --version=1.2.0

nx g @nrwl/nx-plugin:migration fix-files-1.2.0 --project=ionic-react --version=1.2.0

[devinshoemaker]

Update: small e2e test improvement, and rebased onto latest master.

[Cammisuli]

Thanks @devinshoemaker! Everything looks good, but there's one thing to make sure this works well for everyone.

Since migrations.json is a new asset file, you're going to have to modify the assets property in the workspace.json to include the migrations file (similar to builders.json and collection.json).

Also, we removed docs for web, so you might want to rebase on master and regenerate. (Delete the web doc if it doesn't remove it)

[devinshoemaker]

Since migrations.json is a new asset file, you're going to have to modify the assets property in the workspace.json to include the migrations file (similar to builders.json and collection.json).

Do you want the same kind of change to be applied in #2708?

[devinshoemaker]

migrations.json is added by default to a generated nx-plugin, so it sounds like you want to support adding migrations to libs that weren't generated with nx-plugin. I'm happy to make the change, but if you want the "schematic" schematic to work the same way then it sounds like I would need to make a similar change over there.

EDIT
Never mind, I misread the code at first and am on the same page now. 👍

[devinshoemaker]

Requested changes made, rebased, and documentation regenerated.

[Cammisuli]

LGTM!

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