Provide perspective token in JupyterLab to ensure proper ordering of other extensions

[timkpaine]

JupyterLab has a mechanism for having plugins depend on other plugins.

Since perspective registers custom elements (see perspective-dev#3056), this change lets other Jupyter extensions "depend" on @finos/perspective-jupyterlab by doing:

import {IPerspective} from "@finos/perspective-jupyterlab";

export const MyCoolPlugin = {
  activate,
  id: "my-cool-plugin",
  requires: [IPerspective],
  autoStart: true,
};

This then ensures that @finos/perspective-jupyterlab gets activated before MyCoolPlugin, ensuring that custom webcomponents and wasm loading occurs. Downstream plugins like MyCoolPlugin can then just do:

import perspective from "@finos/perspective";

without any additional initialization.

Fixes perspective-dev#3056

I'm open to adding tests but its just a constant, not sure what the best mechanism for doing so is.
We can also add developer docs if we want, happy to write them. done

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