Adding an entry to the TabItemTokens to make the text adjustable in size

[CarlosUrbina]

Problem

Proposed change to address issue #662, adding a new entry in TabItemTokens so clients can change the Text Size

Root cause

There's no entry in TabItemTokens to change the font size

Fix

Added an entry to TabItemTokens, using the same default as TextStyle which is the current option. With that, clients can change the text size for tab entries

Validations

Validated change compile, does not affect existent overrides.

Screenshots

Before	After
	Example with size = 8.sp

Pull request checklist

This PR has considered:

• [x] Light and Dark appearances
• [ ] Automated Tests
• [x] Documentation and demo app examples
• [x] VoiceOver and Keyboard Accessibility
• [x] Internationalization and RTL layouts
• [x] Size classes and window sizes (notched devices, multitasking, different window sizes, etc)

[Anubhvv]

A fix for this issue has already been merged: https://github.com/microsoft/fluentui-android/pull/667