[web] Add tinycolor2 to web/package.json

Summary: So we can use tinycolor.equals(a, b) in ColorSelectorButton

Test Plan: Tested implicitly by D3701

Reviewers: palys-swm, def-au1t, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3700