[wg/webextensions] Update Coordination section

[csarven]

To ensure that the Web Extensions WG coordinates adequately with other groups and organisations, the charter should include or clarify relevant coordination points. The following groups should be considered for addition to the Coordination section for W3C Groups.

W3C Groups

• FedCM Working Group
• Threat Modeling Community Group
• Web Applications Security Working Group
• Web Applications Working Group
• Web Authentication Working Group