Migrate to ECMAScript Modules

Migrate the usage of CommonJS specific code and imports to ESM.  Roughly
summarised this:
- Changes all require() calls to import statement or expressions.
- Changes usage of module.exports to export statements.
- Replaces require.resolve with import.meta.resolve.
- Replaces conditional imports with conditional subpath imports.
- Replaces module === require.main with import.meta.main.
- Adds the full filename with extention to all relative imports.
- Changes all usage of __dirname and __filename to their import.meta
  equivalents.
- Sets "type": "module" in package.json files.
- Removes usage of "use strict".
- Adds .ts an an alias of .js for the webpack build.

Due to differences in how require and import resolves files the plugin
entrypoints will have to include the filename and extension after this
migration in order to work.

docs/devs/json-serialisable-classes.md

@@ -14,7 +14,7 @@ See the following example code for a hypothetical `Data` class with a single pro
  * A Data class example
  * @alias module:Data
  */
-class Data {
+export class Data {
     /** @type {string} */
     member = "foo";
 
@@ -41,8 +41,6 @@ class Data {
         return json;
     }
 }
-
-module.exports = Data;
 ```
 
 Serialising an instance of `Data` for JSON is as simple as doing `serialised = JSON.stringify(data)` as the `toJSON` method is automatically called by `JSON.stringify` to create a suitable object.
@@ -53,14 +51,13 @@ For example a class holding a data property with an instance of the Data class f
 
 ```js
 // Composed.js
-"use strict";
-const Data = require("./Data");
+import Data from "./Data";
 
 /**
  * Composed data class example
  * @alias module:Data
  */
-class Composed {
+export class Composed {
     /** @type {module:Data|undefined} */
     data;
 
@@ -83,8 +80,6 @@ class Composed {
 
     // toJSON is optional and not needed for this particular class
 }
-
-module.exports = Composed;
 ```
 
 It's easy to create a new instance of Composed by calling `new Composed(new Data("spam"))` and serialising to and from JSON is the same as for Data.