fix: move the Session type to Client.ts

[Rexogamer]

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• [x] I understand and have followed the contribution guide
• [x] I have tested my changes locally and they are working as intended
• [x] These changes do not have any notable side effects on other Revolt projects

This PR moves the Session type to Client.ts, fixing an issue where the type wouldn't be available in built versions of revolt.js (Globals.d.ts wasn't included in them).