Implement MessageCommand Interfaces per Enumeration

### Issue
The `MessageCommand` enumeration represents the potential message types that are communicated back and forth, via websockets, between the _dataEditorClient_ object and its WebViewPanel.

Svelte components append an event listener to the DOM that listen for different enumerations of `MessageCommand` and act accordingly. Though, developers are unable to confidently index into the `msg.data.data` object because that portion of the message is always initialized inline.

**Example**
```ts
vscode.postMessage({
command: MessageCommand.testCommand ,
data: {
a: 'a',
b: 'b',
}
})
```

Because of this, there is no index / intellisense relating to the data object.

### Solution
Create and implement an interface, type, or data structure that would allow a developer to type cast the data portion of the VSCode message and accurately determine the structure of each `MessageCommand` enumeration.

**Example**
Receiving a message.
```ts
window.addEventListener('message', (msg) => {
switch (msg.data.command) {
case MessageCommand.test:
const data = msg.data.data as TestCommand
...
```