feat: Add integration to track sessions as app in foreground

[timfish]

Closes #712

This PR

• Adds the BrowserWindowSession integration which tracks sessions as when the app has focus
• There is a timout to ensure that sessions are not ended immediatly if a user briefly switches away from the app
  • The default timeout is 30 seconds but this is configurable via the backgroundTimeoutSeconds constructor option.
• The session on exit handling was moved to src/main/sessions.ts since this is now use by both session integrations
• Some changes to the default integration logic were required to ensure only one session integration is used
• Errors or crashes while the app is in the background will not be attributed to any session

const { init, Integrations } = require('@sentry/electron');

init({
  dsn: '__DSN__',
  integrations: [new Integrations.BrowserWindowSession({ backgroundTimeoutSeconds: 10 })],
});

[timfish]

I've limited this new feature to Electron >= v12 since older versions of Electron seem to unreliably report focus events which make this quite unreliable for sessions.

Electron v12 has been EOL for nearly 2 years now and nobody should be using it for new projects!

[AbhiPrasad]

Errors or crashes while the app is in the background will not be attributed to any session

Let me clarify with the mobile SDKs about what they do here.

[AbhiPrasad]

would be nice to get a test for errored session as well.

[AbhiPrasad]

Errors or crashes while the app is in the background will not be attributed to any session

The behaviour we kept here is good to go!