New plugin api

[texodus]

<perspective-viewer>'s current plugin API is implemented via a configuration object literal, registered via a global registration callback function registerPlugin(). Some deficiencies of this design include:

• Plugin only has plugin_config attribute in public API.
• Hard to style since it is encapsulated in the Shadow DOM, must frequently create new css var aliases.
• Even for plugins that implement light DOM rendering like @finos/perspective-viewer-datagrid, developers must endure a complex setup of awaiting the plugin to render in order to customize it.
• Plugin keys are not discoverable in the UI.

This PR refactors the plugin API to use Custom Elements, rather than configuration object literals, as the plugin template. Plugin Custom Elements reside as the <perspective-viewer>'s light DOM children, which makes it convenient for plugins to expose their own APIs. For example, The <perspective-viewer-debug> element, which just renders the View as a CSV non-virtually here, and has this basic framework:

class DebugPlugin extends HTMLElement {
     get name() {
          return 'Debug';
     }

     get custom_func(x) {
          console.log(`${x} called a custom API call!`);
     }

     static custom_all_funcs(y) {
          console.log(`${x} called a static custom API call!`);
     }

     async draw(view) {
          const csv = await view.to_csv();
          this.innerHTML = `<pre>${csv}</pre>`;
     }
}   

customElements.define('perspective-viewer-debug', DebugPlugin);
registerPlugin("perspective-viewer-debug");

which can be customized at runtime like so:

// Specific instance
const plugin = document.querySelectorAll("perspective-viewer-debug");
plugin.custom_func("Bob Developer");

// All instances
const plugin_class = customElements.get("perspective-viewer-debug");
plugin_class.custom_all_funcs("Alice Developer");

// Customize plugin
class CustomPlugin extends customElements.get("perspective-viewer-debug") {
     async draw(view) {
          const csv = await view.to_csv();
          this.innerHTML = `<title>Custom CSV!!!</title><pre>${csv}</pre>`;
     }
}

customElements.define('perspective-viewer-custom', CustomPlugin);
registerPlugin("perspective-viewer-custom");

Other effects of this PR include:

• type and name have been replaced by just name; e.g., "x_bar" type is now "X Bar", the name plugin property. Docs, examples, python, etc have been updated to reflect this.