Fix debug session to be deleted in $sessionDidDestroy instead of $terminateDebugSession

[Bolodya1997]

What it does

Related to #10953.

Cause

$terminateDebugSession always comes before $sessionDidDestroy and removes debug session from sessions.
As a result, $sessionDidDestroy never fires onDidTerminateDebugSessionEmitter, because there is no session at the moment of the execution.

Solution

$terminateDebugSession shouldn't delete debug session, it should only terminate (stop) it - it comes from the method name.
$sessionDidDestroy should delete debug session - it also comes from the method name.

How to test

1. Create test plugin:

import * as vscode from "vscode";
import { ExtensionContext, window } from "vscode";

export async function activate(_: ExtensionContext) {
  vscode.debug.onDidStartDebugSession((session: vscode.DebugSession) => {
    window.showInformationMessage(`Started debug session: ${session.id}`);
  });

  vscode.debug.onDidTerminateDebugSession((session: vscode.DebugSession) => {
    window.showInformationMessage(`Terminated debug session: ${session.id}`);
  });
}

3. Run Theia with any language supported (tested with Python) with test plugin added.
4. Create any debug configuration.
   
5. Run debug session.
   

Expected behavior

1. Information message 'Started debug session' is shown on execution start.
2. Information message 'Terminated debug session' is shown on execution end.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[Bolodya1997]

@vince-fugnitto @msujew
Please add some comments on this :)
Probably there is someone else who I can mention to get this PR reviewed?

[msujew]

Thank you for your first contribution @Bolodya1997!

I can confirm that the issue exists on master and is fixed by these changes 👍