Migrate `activate()` and `deactivate()` logic into `src/activation/`

We can't safely test anything that requires importing from `extension.ts`, so we should move the logic out to https://github.com/confluentinc/vscode/tree/main/src/activation (possibly into separate files for maintenance/sanity benefits), and then tests should be able to import safely from there. (Which will also make `extension.ts` much smaller.)

Only other change we may want to add is when moving `deactivate()`, adding something to the effect of:
```ts
async extensionDeactivation() {
// (normal deactivate() logic here)

if (context.extensionMode === vscode.ExtensionMode.Test) {
// explicitly dispose everything that would automatically cleanup on a real deactivate() call
getExtensionContext().subscriptions.forEach((d: vscode.Disposable) => d.dispose());
}
}
```