Replace row_pivots with group_by, column_pivots with split_by, add convert() migration utility

[texodus]

Renames row_pivots to group_by, and column_pivots to split_by, and adds a new method convert() to the @finos/perspective-viewer package which can auto-update layouts to the new format. This is a breaking change, but trivial to mitigate via convert()

From the documentation for convert():

convert(old, options)

A migration utility for @finos/perspective-viewer and @finos/perspective-workspace persisted state objects. If you have an application which persists tokens returned by the .save() method of a Perspective Custom Element, and you want to upgrade Perspective to the latest version, this module is for you! You know who you are!

Say you have a <perspective-viewer> Custom Element from @finos/perspective-viewer>=0.8.3, and have persisted an arbitrary persistence token object:

const old_elem = document.querySelector("perspective-viewer");
const old_token = await old_elem.save();

To migrate this token to the version of @finos/perspective-migrate itself:

import {convert} from "@finos/perspective-viewer";

// ...

const new_elem = document.querySelector("perspective-viewer");
const new_token = convert(old_token);
await new_elem.restore(new_token);