feat: SessionTiming integration

[kamilogorek]

Fixes: #1823

Usage:

import { SessionTiming } from '@sentry/integration';

Sentry.init({
  integrations: [new SessionTiming()]
});

[getsentry-bot]

	Fails
🚫	TSLint failed: @sentry/browser ERROR: /home/travis/build/getsentry/sentry-javascript/packages/browser/src/integrations/trycatch.ts[73, 22]: Avoid referencing unbound methods which may cause unintentional scoping of 'this'. ERROR: /home/travis/build/getsentry/sentry-javascript/packages/browser/src/integrations/trycatch.ts[73, 15]: Expression is always true.

	Warnings
⚠️	Please add a changelog entry for your changes.

	Messages
📖	@sentry/browser bundle gzip'ed minified size: (ES5: 15.916 kB) (ES6: 15.1221 kB)

Generated by 🚫 dangerJS against e6bd3b9

[HazAT]

Solid integration 👍

The lint issues are unrelated to this PR but still here.

Also, Changelog. Otherwise 💯