Enable TypeScript compiler's strict mode

### Prerequisites

- [X] I have written a descriptive issue title.
- [X] I have searched all [issues](https://.com/PowerShell/vscode-powershell/issues?q=is%3Aissue) to ensure it has not already been reported.

### Summary

The following options should be enabled in [`tsconfig.json`](https://.com/PowerShell/vscode-powershell/blob/master/tsconfig.json):

```json
"strict": true,
"noImplicitReturns": true,
"noFallthroughCasesInSwitch": true,
"noUnusedParameters": true
```

### Proposed Design

We'll have to carefully fix all the errors and potential bugs that this reveals.

Description

Prerequisites

Summary

Proposed Design